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/input.rs

@@ -407,6 +407,10 @@ pub(super) fn handle_settings_buttons(
             | SettingsButton::DeleteAccount => {
                 // Handled by `handle_sync_buttons`.
             }
+            #[cfg(not(target_arch = "wasm32"))]
+            SettingsButton::OpenThemeStore => {
+                // Handled by `handle_sync_buttons`.
+            }
             SettingsButton::Done => {
                 screen.0 = false;
             }
@@ -423,6 +427,9 @@ pub(super) fn handle_sync_buttons(
     mut configure_sync: MessageWriter<SyncConfigureRequestEvent>,
     mut logout_sync: MessageWriter<SyncLogoutRequestEvent>,
     mut delete_account: MessageWriter<DeleteAccountRequestEvent>,
+    #[cfg(not(target_arch = "wasm32"))] mut open_theme_store: MessageWriter<
+        crate::events::ThemeStoreOpenRequestEvent,
+    >,
     mut screen: ResMut<SettingsScreen>,
 ) {
     for (interaction, button) in &interaction_query {
@@ -439,6 +446,13 @@ pub(super) fn handle_sync_buttons(
                 screen.0 = false;
                 configure_sync.write(SyncConfigureRequestEvent);
             }
+            #[cfg(not(target_arch = "wasm32"))]
+            SettingsButton::OpenThemeStore => {
+                // Same shape as ConnectSync: close settings first so the
+                // store modal's own-scrim guard doesn't block.
+                screen.0 = false;
+                open_theme_store.write(crate::events::ThemeStoreOpenRequestEvent);
+            }
             SettingsButton::DisconnectSync => {
                 logout_sync.write(SyncLogoutRequestEvent);
             }