feat(web): persist game state across page refreshes with resume dialog

- solitaire_wasm: add SolitaireGame::serialize() and from_saved() so JS
  can round-trip the full GameState through localStorage as JSON
- game.js: save {gameState, elapsedSecs, drawThree} to localStorage
  (key: fs_game_save) on every render(); clear the save on win
- game.js: on bootstrap, check for a saved game and show a resume
  dialog if one exists; Resume restores state + timer, New Game discards
  the save and starts fresh with a random seed
- game.html: add #resume-overlay markup (same pattern as win-overlay)
- game.css: add styles for the resume dialog and its secondary button

localStorage failures (private-browsing quota) are silently ignored so
they never block gameplay.

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

solitaire_server/web/game.html

@@ -56,6 +56,17 @@
         </section>
     </main>
 
+    <div id="resume-overlay" class="hidden">
+        <div class="resume-card">
+            <div class="resume-title">Resume Game?</div>
+            <p class="resume-detail">You have an unfinished game saved. Would you like to continue where you left off?</p>
+            <div class="resume-actions">
+                <button id="btn-resume">↩ Resume</button>
+                <button id="btn-resume-new" class="secondary">↺ New Game</button>
+            </div>
+        </div>
+    </div>
+
     <div id="win-overlay" class="hidden">
         <div class="win-card">
             <div class="win-title">You Won!</div>