Add chapter checkpoint feature

- New feature: bookmark messages to exclude earlier history from context
- Saves tokens by marking chapter start points in long chats
- Uses SillyTavern's /hide and /unhide slash commands
- Persists checkpoint across page reloads and generation events
- UI: bookmark icon in message menus with visual indicators
- Debounced restore function prevents concurrent executions
- Pre-generation checkpoint application ensures messages stay hidden
- Clean production-ready code with proper error handling

style.css

@@ -6637,3 +6637,54 @@ body:has(.rpg-panel.rpg-position-left) #sheld {
         font-size: clamp(14px, 3vw, 18px) !important;
     }
 }
+
+/* ============================================
+   CHAPTER CHECKPOINT STYLES
+   ============================================ */
+
+/* Checkpoint indicator in messages */
+.rpg-checkpoint-indicator {
+    display: flex;
+    align-items: center;
+    gap: 8px;
+    padding: 8px 12px;
+    margin-bottom: 12px;
+    background: linear-gradient(135deg, rgba(74, 158, 255, 0.15) 0%, rgba(74, 158, 255, 0.05) 100%);
+    border-left: 4px solid #4a9eff;
+    border-radius: 4px;
+    font-size: 13px;
+    font-weight: 600;
+    color: #4a9eff;
+    animation: checkpoint-fade-in 0.3s ease-out;
+}
+
+.rpg-checkpoint-indicator i {
+    font-size: 16px;
+}
+
+@keyframes checkpoint-fade-in {
+    from {
+        opacity: 0;
+        transform: translateX(-10px);
+    }
+    to {
+        opacity: 1;
+        transform: translateX(0);
+    }
+}
+
+/* Optional: Add a subtle glow effect for checkpoint messages */
+.mes:has(.rpg-checkpoint-indicator) {
+    position: relative;
+}
+
+.mes:has(.rpg-checkpoint-indicator)::before {
+    content: '';
+    position: absolute;
+    left: -4px;
+    top: 0;
+    bottom: 0;
+    width: 3px;
+    background: linear-gradient(180deg, #4a9eff 0%, transparent 100%);
+    opacity: 0.5;
+}