feat: P2 - QTE system, subtitles, save thumbnails

- QTESystem: trigger detection via timeupdate, multi-key matching, timeout handling
- QTEOverlay: SVG countdown ring + key prompts + success/fail animation
- Engine: integrate QTE (timeupdate check, conditional branching, effect application)
- Subtitles: WebVTT parsing + synchronized subtitle rendering
- GamePlayer: overlay QTE and subtitle components
- SaveSystem: DB v2 with thumbnail field, canvas snapshot at 320x180 JPEG
- SaveLoadMenu: thumbnail preview for save slots
- VideoManager: getActiveVideoElement() for canvas capture
- App.vue: QTE/subtitle integration, thumbnail capture on save
- stores: QTE state management, save list with thumbnails
- demo.json: QTE scene (right_door), subtitles, new event types
- ROADMAP: mark P2 as completed

engine/systems/SaveSystem.ts

@@ -9,6 +9,7 @@ interface SaveRecord {
   variables: string
   flags: string
   history: string
+  thumbnail?: string
 }
 
 class SaveDB extends Dexie {
@@ -16,7 +17,7 @@ class SaveDB extends Dexie {
 
   constructor() {
     super('MovieGameSaves')
-    this.version(1).stores({
+    this.version(2).stores({
       saves: '++id, slot',
     })
   }
@@ -25,14 +26,15 @@ class SaveDB extends Dexie {
 const db = new SaveDB()
 
 export class SaveSystem {
-  async save(slot: number, data: Omit<SaveData, 'slot' | 'thumbnail'>): Promise<void> {
+  async save(slot: number, data: Omit<SaveData, 'slot'>): Promise<void> {
     const record: SaveRecord = {
       slot,
-      timestamp: Date.now(),
+      timestamp: data.timestamp || Date.now(),
       currentScene: data.currentScene,
       variables: JSON.stringify(data.variables),
       flags: JSON.stringify(data.flags),
       history: JSON.stringify(data.history),
+      thumbnail: data.thumbnail,
     }
 
     const existing = await db.saves.where('slot').equals(slot).first()
@@ -54,15 +56,17 @@ export class SaveSystem {
       variables: JSON.parse(record.variables),
       flags: JSON.parse(record.flags),
       history: JSON.parse(record.history),
+      thumbnail: record.thumbnail,
     }
   }
 
-  async listSlots(): Promise<{ slot: number; timestamp: number; sceneLabel: string }[]> {
+  async listSlots(): Promise<{ slot: number; timestamp: number; sceneLabel: string; thumbnail?: string }[]> {
     const records = await db.saves.orderBy('slot').toArray()
     return records.map((r) => ({
       slot: r.slot,
       timestamp: r.timestamp,
       sceneLabel: r.currentScene,
+      thumbnail: r.thumbnail,
     }))
   }