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/assets/mod.rs

@@ -11,8 +11,8 @@ pub mod svg_loader;
 pub mod user_dir;
 
 pub use sources::{
-    default_theme_svg_bytes, populate_embedded_default_theme, register_theme_asset_sources,
-    AssetSourcesPlugin, DEFAULT_THEME_MANIFEST_URL, USER_THEMES,
+    bundled_theme_url, dark_theme_svg_bytes, populate_embedded_dark_theme,
+    register_theme_asset_sources, AssetSourcesPlugin, DARK_THEME_MANIFEST_URL, USER_THEMES,
 };
 pub use svg_loader::{rasterize_svg, SvgLoader, SvgLoaderError, SvgLoaderSettings};
 pub use user_dir::{set_user_theme_dir, user_theme_dir};