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/clubs_3.svg

@@ -0,0 +1,25 @@
+<svg xmlns="http://www.w3.org/2000/svg" width="256" height="384" viewBox="0 0 256 384">
+  <rect x="0" y="0" width="256" height="384" rx="16" ry="16"
+        fill="#1a1a1a"/>
+
+  <!-- Top-left rank in JetBrains-Mono-styled FiraMono (rank digits
+       and letters render correctly in FiraMono; only the suit glyphs
+       needed to escape to paths). -->
+  <text x="14" y="44" font-family="Fira Mono" font-size="36" font-weight="700"
+        fill="#e8e8e8">3</text>
+
+  <!-- Top-left small suit glyph at (14, 50), 20 × 20.
+       `suit_path_d` is authored in a 32-unit box, so scale 0.625
+       lands the visible glyph at 20 px. -->
+  <g transform="translate(14 50) scale(0.625)">
+    <path d="M16,4 C 13,4 10,7 10,10 C 10,12 11,13 12,14 C 9,14 4,17 4,21 C 4,24 7,27 10,27 C 12,27 14,26 14,24 L 13,30 L 19,30 L 18,24 C 18,26 20,27 22,27 C 25,27 28,24 28,21 C 28,17 23,14 20,14 C 21,13 22,12 22,10 C 22,7 19,4 16,4 Z" fill="none" stroke="#e8e8e8" stroke-width="3"/>
+  </g>
+
+  <!-- Bottom-right large suit glyph at (178, 286), 64 × 64.
+       Visible bottom-right at (242, 350), visible top-left at
+       (178, 286). Same upright orientation as the top-left small
+       glyph — no 180° rotation applied. -->
+  <g transform="translate(178 286) scale(2)">
+    <path d="M16,4 C 13,4 10,7 10,10 C 10,12 11,13 12,14 C 9,14 4,17 4,21 C 4,24 7,27 10,27 C 12,27 14,26 14,24 L 13,30 L 19,30 L 18,24 C 18,26 20,27 22,27 C 25,27 28,24 28,21 C 28,17 23,14 20,14 C 21,13 22,12 22,10 C 22,7 19,4 16,4 Z" fill="none" stroke="#e8e8e8" stroke-width="3"/>
+  </g>
+</svg>