feat(engine): rename themes — Classic is default, Dark replaces Default

- Rename assets/themes/default/ → assets/themes/dark/; update theme.ron
  id/name to "dark"/"Dark"
- Rename all DEFAULT_THEME_* constants → DARK_THEME_* and
  default_theme_svg_bytes / populate_embedded_default_theme → dark_*
- Add bundled_theme_url() helper for URL resolution without needing the
  registry (used by Startup systems where ordering isn't guaranteed)
- Registry now lists Classic first (new player default), Dark second
- settings.rs default_theme_id() returns "classic" so fresh installs
  start on the white card theme

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

solitaire_engine/src/lib.rs

@@ -53,8 +53,8 @@ pub mod weekly_goals_plugin;
 pub mod win_summary_plugin;
 
 pub use assets::{
-    populate_embedded_default_theme, register_theme_asset_sources, AssetSourcesPlugin,
-    DEFAULT_THEME_MANIFEST_URL, USER_THEMES,
+    bundled_theme_url, populate_embedded_dark_theme, register_theme_asset_sources,
+    AssetSourcesPlugin, DARK_THEME_MANIFEST_URL, USER_THEMES,
 };
 pub use theme::{
     set_theme, ActiveTheme, CardTheme, CardThemeLoader, ThemeEntry, ThemePlugin, ThemeRegistry,