feat(web): add classic/dark card theme picker

- Reorganise card PNGs into assets/cards/faces/{classic,dark}/ and
  assets/cards/backs/{classic,dark}/
- Rasterise dark SVG theme alongside existing classic set
- Add "Dark / Classic" toggle button in the game HUD; persists to
  localStorage as fs_theme (defaults to classic)
- Preload both themes on page load so switching is instant

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

solitaire_server/web/game.html

@@ -39,6 +39,7 @@
             <label class="toggle-label" title="Draw one or three cards">
                 <input type="checkbox" id="chk-draw3"> Draw 3
             </label>
+            <button id="btn-theme" title="Switch card theme">Dark</button>
         </div>
     </header>