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.css

@@ -230,6 +230,68 @@ main {
     pointer-events: none;
 }
 
+/* ── Resume overlay ──────────────────────────────────────────────────── */
+
+#resume-overlay {
+    position: fixed;
+    inset: 0;
+    background: rgba(21, 21, 21, 0.92);
+    display: flex;
+    align-items: center;
+    justify-content: center;
+    z-index: 1000;
+}
+
+#resume-overlay.hidden { display: none; }
+
+.resume-card {
+    background: var(--panel);
+    border: 1px solid rgba(255,255,255,0.12);
+    border-radius: 16px;
+    padding: 40px 48px;
+    text-align: center;
+    display: flex;
+    flex-direction: column;
+    gap: 16px;
+    box-shadow: 0 20px 60px rgba(0,0,0,0.6);
+    max-width: 360px;
+}
+
+.resume-title {
+    font-size: 28px;
+    font-weight: 700;
+    color: var(--accent);
+}
+
+.resume-detail {
+    font-size: 14px;
+    color: var(--text-muted);
+    margin: 0;
+    line-height: 1.5;
+}
+
+.resume-actions {
+    display: flex;
+    gap: 12px;
+    justify-content: center;
+    margin-top: 8px;
+}
+
+.resume-actions button {
+    padding: 12px 24px;
+    font-size: 15px;
+}
+
+.resume-actions button.secondary {
+    background: transparent;
+    border: 1px solid rgba(255,255,255,0.2);
+    color: var(--text-muted);
+}
+
+.resume-actions button.secondary:hover {
+    background: rgba(255,255,255,0.05);
+}
+
 /* ── Win overlay ─────────────────────────────────────────────────────── */
 
 #win-overlay {

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>

solitaire_server/web/game.js

@@ -69,6 +69,34 @@ function preloadTheme(theme) {
 preloadTheme("classic");
 preloadTheme("dark");
 
+// ── Persistence ──────────────────────────────────────────────────────────────
+const LS_SAVE_KEY = "fs_game_save";
+
+function saveState() {
+    if (!game) return;
+    try {
+        const gameState = game.serialize();
+        if (typeof gameState !== "string") return;
+        localStorage.setItem(LS_SAVE_KEY, JSON.stringify({ gameState, elapsedSecs, drawThree }));
+    } catch (e) {
+        // localStorage may be unavailable (private browsing quota, etc.) — never block gameplay.
+        console.warn("fs: save failed", e);
+    }
+}
+
+function clearSave() {
+    try { localStorage.removeItem(LS_SAVE_KEY); } catch { /* ignore */ }
+}
+
+function loadSave() {
+    try {
+        const raw = localStorage.getItem(LS_SAVE_KEY);
+        if (!raw) return null;
+        const save = JSON.parse(raw);
+        return save?.gameState ? save : null;
+    } catch { return null; }
+}
+
 // ── State ────────────────────────────────────────────────────────────────────
 let game      = null;
 let snap      = null;   // last rendered GameSnapshot
@@ -138,16 +166,72 @@ async function bootstrap() {
     await init();
     syncThemeButton();
 
+    buildSlots();
+    scaleBoard();
+    window.addEventListener("resize", scaleBoard);
+    attachHandlers();
+
+    const saved = loadSave();
+    if (saved) {
+        showResumeDialog(saved);
+    } else {
         const params  = new URLSearchParams(window.location.search);
         const urlSeed = params.has("seed") ? Number(params.get("seed")) : randomSeed();
         drawThree     = params.has("draw3");
         chkDraw3.checked = drawThree;
-
-    buildSlots();
-    scaleBoard();
-    window.addEventListener("resize", scaleBoard);
         startGame(urlSeed);
-    attachHandlers();
+    }
+}
+
+function showResumeDialog(saved) {
+    const overlay = document.getElementById("resume-overlay");
+    if (overlay) overlay.classList.remove("hidden");
+
+    document.getElementById("btn-resume").onclick = () => {
+        if (overlay) overlay.classList.add("hidden");
+        resumeGame(saved);
+    };
+    document.getElementById("btn-resume-new").onclick = () => {
+        clearSave();
+        if (overlay) overlay.classList.add("hidden");
+        drawThree = false;
+        chkDraw3.checked = false;
+        startGame(randomSeed());
+    };
+}
+
+function resumeGame(saved) {
+    let restored;
+    try {
+        restored = SolitaireGame.from_saved(saved.gameState);
+    } catch (e) {
+        console.warn("fs: restore failed, starting new game", e);
+        clearSave();
+        startGame(randomSeed());
+        return;
+    }
+
+    game = restored;
+    drawThree = !!saved.drawThree;
+    elapsedSecs = saved.elapsedSecs || 0;
+    chkDraw3.checked = drawThree;
+
+    const displaySeed = Math.round(game.seed());
+    hudSeed.textContent = `seed ${displaySeed}`;
+    winOverlay.classList.add("hidden");
+    cardEls.clear();
+    board.querySelectorAll(".card, .recycle-label").forEach(el => el.remove());
+
+    const url = new URL(window.location);
+    url.searchParams.set("seed", displaySeed);
+    if (drawThree) url.searchParams.set("draw3", "");
+    else           url.searchParams.delete("draw3");
+    history.replaceState(null, "", url);
+
+    const s = game.state();
+    snap = s;
+    render(s);
+    if (!s.is_won) startTimer();
 }
 
 function randomSeed() {
@@ -304,9 +388,12 @@ function render(s) {
         acTimer = setInterval(doAutoCompleteStep, 380);
     }
     if (s.is_won) {
+        clearSave();
         stopTimer();
         if (acTimer) { clearInterval(acTimer); acTimer = null; }
         showWin(s);
+    } else {
+        saveState();
     }
 }
 

solitaire_wasm/src/lib.rs

@@ -422,6 +422,25 @@ impl SolitaireGame {
         }
     }
 
+    /// Serialise the full game state as a JSON string for `localStorage`.
+    ///
+    /// Use [`SolitaireGame::from_saved`] to restore it. The returned string is
+    /// opaque — callers should treat it as a blob and store/restore it verbatim.
+    pub fn serialize(&self) -> Result<String, JsValue> {
+        serde_json::to_string(&self.game)
+            .map_err(|e| JsValue::from_str(&e.to_string()))
+    }
+
+    /// Restore a game from a JSON string previously produced by [`SolitaireGame::serialize`].
+    ///
+    /// Returns an error string if the JSON is malformed or describes a state
+    /// that can't be deserialised (e.g. from a future schema version).
+    pub fn from_saved(json: &str) -> Result<SolitaireGame, JsValue> {
+        serde_json::from_str::<GameState>(json)
+            .map(|game| SolitaireGame { game })
+            .map_err(|e| JsValue::from_str(&e.to_string()))
+    }
+
     /// Apply one auto-complete move (only valid when `is_auto_completable`).
     ///
     /// If no card can go directly to a foundation this step, advances the