fix(engine,wasm,web): detect no-legal-moves correctly and surface banner

Engine: replace broken has_legal_moves loop (which checked buried
mid-column cards without sequence validation) with a delegation to
possible_instructions(), mirroring the hint system's logic exactly.

WASM: add has_moves: bool to GameSnapshot, computed in snap() using the
same stock/waste/possible_instructions check so the web client gets the
flag in every state update at no extra round-trip cost.

Web: show a non-blocking no-moves banner (slide-up toast) with Undo and
New Game actions when has_moves is false and the game is not won. Banner
hides automatically once a move restores legal play (e.g. after undo).

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

solitaire_server/web/game.html

@@ -77,6 +77,17 @@
         </div>
     </div>
 
+    <div id="no-moves-banner" class="hidden">
+        <div class="no-moves-card">
+            <div class="no-moves-title">No Moves Available</div>
+            <p class="no-moves-detail">No legal moves remain. Undo to go back or start a new game.</p>
+            <div class="no-moves-actions">
+                <button id="btn-no-moves-undo">↩ Undo</button>
+                <button id="btn-no-moves-new" class="secondary">↺ New Game</button>
+            </div>
+        </div>
+    </div>
+
     <script type="module" src="/web/game.js"></script>
 </body>
 </html>