feat: auto-save replays on game over and page unload

- Add autoSave() function that saves replay silently on game over
- Add beforeunload listener to save incomplete games when navigating away
- Add prominent 'Save Now' button in status bar (visible during play)
- Show 'Replay auto-saved!' notification in status bar
- Add gameSaved flag to prevent duplicate saves

src/blokus_ui/static/style.css

@@ -342,6 +342,24 @@ h3 {
     font-weight: bold;
     font-size: 1.05em;
     min-height: 24px;
+    display: flex;
+    justify-content: space-between;
+    align-items: center;
+}
+
+#status-save-btn {
+    padding: 4px 12px;
+    background: var(--blue);
+    color: var(--bg);
+    border: none;
+    border-radius: 4px;
+    cursor: pointer;
+    font-size: 0.9em;
+    font-weight: bold;
+}
+
+#status-save-btn:hover {
+    background: #5bb3ff;
 }
 
 /* ===== Replay Page ===== */