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.css

@@ -355,6 +355,67 @@ main {
     animation: illegal-shake 320ms ease;
 }
 
+/* ── No-moves banner ─────────────────────────────────────────────────── */
+
+#no-moves-banner {
+    position: fixed;
+    bottom: 24px;
+    left: 50%;
+    transform: translateX(-50%);
+    z-index: 900;
+    animation: slide-up 240ms ease;
+}
+
+#no-moves-banner.hidden { display: none; }
+
+.no-moves-card {
+    background: var(--panel);
+    border: 1px solid rgba(255,255,255,0.15);
+    border-radius: 12px;
+    padding: 20px 32px;
+    text-align: center;
+    display: flex;
+    flex-direction: column;
+    gap: 12px;
+    box-shadow: 0 8px 32px rgba(0,0,0,0.7);
+    min-width: 300px;
+}
+
+.no-moves-title {
+    font-size: 18px;
+    font-weight: 700;
+    color: var(--accent);
+}
+
+.no-moves-detail {
+    font-size: 13px;
+    color: var(--text-muted);
+    margin: 0;
+    line-height: 1.5;
+}
+
+.no-moves-actions {
+    display: flex;
+    gap: 10px;
+    justify-content: center;
+    margin-top: 4px;
+}
+
+.no-moves-actions button.secondary {
+    background: transparent;
+    border: 1px solid rgba(255,255,255,0.2);
+    color: var(--text-muted);
+}
+
+.no-moves-actions button.secondary:hover {
+    background: rgba(255,255,255,0.05);
+}
+
+@keyframes slide-up {
+    from { opacity: 0; transform: translateX(-50%) translateY(12px); }
+    to   { opacity: 1; transform: translateX(-50%) translateY(0); }
+}
+
 /* ── Foundation slot suit hints ──────────────────────────────────────────── */
 
 .slot-hint {