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/assets/themes/dark/theme.ron

@@ -13,9 +13,9 @@
 //   ranks: ace, 2, 3, 4, 5, 6, 7, 8, 9, 10, jack, queen, king
 (
     meta: (
-        id: "default",
-        name: "Default",
-        author: "Solitaire Quest",
+        id: "dark",
+        name: "Dark",
+        author: "Ferrous Solitaire",
         version: "0.1.0",
         card_aspect: (2, 3),
     ),