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/mod.rs

@@ -250,6 +250,9 @@ enum SettingsButton {
     /// Scan `user_theme_dir()` for new `.zip` files and import each one.
     #[cfg(not(target_arch = "wasm32"))]
     ScanThemes,
+    /// Open the in-game theme-store modal (closes Settings first).
+    #[cfg(not(target_arch = "wasm32"))]
+    OpenThemeStore,
     SyncNow,
     /// Open the sync-server Connect modal (shown when backend = Local).
     ConnectSync,
@@ -313,6 +316,8 @@ impl SettingsButton {
             SettingsButton::SelectTheme(_) => 85,
             #[cfg(not(target_arch = "wasm32"))]
             SettingsButton::ScanThemes => 86,
+            #[cfg(not(target_arch = "wasm32"))]
+            SettingsButton::OpenThemeStore => 87,
             // Sync section
             SettingsButton::SyncNow => 90,
             SettingsButton::ConnectSync => 91,
@@ -371,6 +376,7 @@ impl Plugin for SettingsPlugin {
             .add_message::<SyncLogoutRequestEvent>()
             .add_message::<DeleteAccountRequestEvent>()
             .add_message::<ToggleSettingsRequestEvent>()
+            .add_message::<crate::events::ThemeStoreOpenRequestEvent>()
             .add_message::<InfoToastEvent>()
             .add_message::<MouseWheel>()
             .add_message::<TouchInput>()