feat: in-game theme store — server catalog + verified downloads + install UI

Phase 1+2 of the theme-store roadmap: a free catalog served by
solitaire_server and an in-app browse/install flow, making custom
themes installable on Android for the first time (the manual
drop-a-zip flow can't reach the app-private themes dir there).

- solitaire_sync: ThemeCatalogEntry/ThemeCatalogResponse wire types
  (additive module; SyncPayload and SyncProvider untouched)
- solitaire_server: THEME_STORE_DIR scan at startup (meta-only
  theme.ron parse, sha256, 20 MiB cap, best-effort per archive);
  public GET /api/themes, /api/themes/{id}/download, /{id}/preview;
  compose volume + README_SERVER docs
- solitaire_data: ThemeStoreClient — catalog fetch + download with
  mandatory size/sha256 verification before bytes are released
- solitaire_engine: ThemeStorePlugin — 'Browse theme store' button in
  Settings → Cosmetic, modal catalog (leaderboard-style rebuild),
  download on AsyncComputeTaskPool, atomic .tmp+rename write into
  user_theme_dir, then the existing hardened import_theme pipeline and
  an in-place registry refresh

New deps: sha2 (workspace, server+data); ron/zip reused in server;
serde_json added to solitaire_sync dev-deps for DTO round-trip tests.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

solitaire_engine/src/settings_plugin/ui.rs

@@ -239,6 +239,8 @@ pub(super) fn spawn_settings_panel(
             }
             #[cfg(not(target_arch = "wasm32"))]
             import_themes_row(body, font_res);
+            #[cfg(not(target_arch = "wasm32"))]
+            theme_store_row(body, font_res);
 
             // --- Privacy (only shown when a Matomo URL is configured) ---
             if settings.matomo_url.is_some() {
@@ -1193,6 +1195,31 @@ pub(super) fn import_themes_row(
         });
 }
 
+/// "Theme store" row: one pill button that closes Settings and opens
+/// the in-game theme-store modal (`theme_store_plugin`). Sits directly
+/// under the Import row so both install paths live together.
+#[cfg(not(target_arch = "wasm32"))]
+pub(super) fn theme_store_row(parent: &mut ChildSpawnerCommands, font_res: Option<&FontResource>) {
+    parent
+        .spawn((
+            FocusRow,
+            Node {
+                flex_direction: FlexDirection::Row,
+                align_items: AlignItems::Center,
+                ..default()
+            },
+        ))
+        .with_children(|row| {
+            pill_button(
+                row,
+                SettingsButton::OpenThemeStore,
+                "Browse theme store",
+                "Download themes from your sync server without leaving the game.",
+                font_res,
+            );
+        });
+}
+
 pub(super) fn icon_button(
     parent: &mut ChildSpawnerCommands,
     label: &str,