feat(engine): card-art thumbnails in the theme picker

Settings → Cosmetic's theme picker showed only the theme name. Now
each chip carries a small Ace-of-Spades + back preview pair so the
player can see what each theme looks like before switching.

A new ThemeThumbnailCache resource keys per-theme by id and stores
two Handle<Image>s (ace + back) rasterised at thumbnail resolution
via the existing rasterize_svg path. Generation runs once per
theme registration in theme_plugin; subsequent picker re-spawns
just look up the cached handles. Themes that lack one of the
preview SVGs (broken user theme) get a Handle::default() placeholder
rather than crashing — the placeholder renders as a transparent
rectangle the same size as the missing thumbnail.

The picker chip spawn loop in settings_plugin reads the cache and
renders the pair as two child sprites above the chip text. The
selected-theme chip's existing STATE_SUCCESS tint sits behind the
thumbnails; contrast stays readable.

Asset-source plumbing in assets/sources.rs and assets/mod.rs picks
up the new bytes-loading helper that the thumbnail generator uses
for embedded:// theme assets at startup time (before AssetServer is
fully initialised).

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

solitaire_engine/src/assets/sources.rs

@@ -194,6 +194,25 @@ impl Plugin for AssetSourcesPlugin {
     }
 }
 
+/// Returns the embedded SVG bytes for a single default-theme file
+/// (e.g. `"back.svg"` or `"spades_ace.svg"`), or `None` when the
+/// filename is not bundled.
+///
+/// The thumbnail generator in
+/// [`crate::theme::ThemeThumbnailCache`] uses this to rasterise
+/// preview-sized art for the picker UI without going through Bevy's
+/// async asset graph. Lookup is by the filename only — the
+/// `solitaire_engine/assets/themes/default/` prefix is stripped before
+/// comparison so callers don't need to know where the embedded files
+/// live in the binary.
+pub fn default_theme_svg_bytes(filename: &str) -> Option<&'static [u8]> {
+    let suffix = format!("/{filename}");
+    DEFAULT_THEME_SVGS
+        .iter()
+        .find(|(path, _)| path.ends_with(&suffix))
+        .map(|(_, bytes)| *bytes)
+}
+
 /// Pushes every bundled default-theme file into the
 /// [`EmbeddedAssetRegistry`] under its stable URL. Keeping this in a
 /// free function (and not inside the `Plugin::build` body) means the
@@ -291,6 +310,29 @@ mod tests {
         assert_eq!(faces.len(), 52);
     }
 
+    /// `default_theme_svg_bytes` resolves the canonical preview pair
+    /// the thumbnail cache rasterises: `back.svg` and `spades_ace.svg`.
+    /// Both must exist in the embedded table or the picker's preview
+    /// thumbnails would silently fall back to placeholders even for the
+    /// always-present default theme.
+    #[test]
+    fn default_theme_svg_bytes_finds_back_and_ace_of_spades() {
+        assert!(
+            default_theme_svg_bytes("back.svg").is_some(),
+            "default theme must bundle a back.svg"
+        );
+        assert!(
+            default_theme_svg_bytes("spades_ace.svg").is_some(),
+            "default theme must bundle a spades_ace.svg"
+        );
+    }
+
+    #[test]
+    fn default_theme_svg_bytes_returns_none_for_unknown_file() {
+        assert!(default_theme_svg_bytes("nope.svg").is_none());
+        assert!(default_theme_svg_bytes("").is_none());
+    }
+
     /// Belt-and-braces: if anyone edits `DEFAULT_THEME_MANIFEST_PATH`
     /// without updating `DEFAULT_THEME_MANIFEST_URL` (or vice versa)
     /// the asset would register at one path and be loaded from