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/back.svg

@@ -0,0 +1,21 @@
+<svg xmlns="http://www.w3.org/2000/svg" width="256" height="384" viewBox="0 0 256 384">
+  <defs>
+    <pattern id="scanlines" x="0" y="0" width="2" height="4" patternUnits="userSpaceOnUse">
+      <rect x="0" y="0" width="2" height="2" fill="#1a1a1a"/>
+    </pattern>
+  </defs>
+
+  <!-- Background fill, then scanlines on top (the scanlines stay
+       darker than BACK_BG so the "off" rows show through). -->
+  <rect x="1" y="1" width="254" height="382" rx="16" ry="16"
+        fill="#151515" stroke="#353535" stroke-width="2"/>
+  <rect x="1" y="1" width="254" height="382" rx="16" ry="16"
+        fill="url(#scanlines)"/>
+
+  <!-- Top-left accent badge (the only theme-varying element). -->
+  <rect x="12" y="12" width="24" height="32" fill="#a54242"/>
+
+  <!-- Bottom-right "▌RS" monogram in JetBrains-Mono-styled FiraMono. -->
+  <text x="244" y="368" font-family="Fira Mono" font-size="24"
+        fill="#505050" text-anchor="end">&#x258C;RS</text>
+</svg>