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/core/VideoManager.ts

@@ -132,6 +132,10 @@ export class VideoManager {
     return this.active?.currentTime ?? 0
   }
 
+  getActiveVideoElement(): HTMLVideoElement | null {
+    return this.active ?? null
+  }
+
   onEnd(cb: VideoEndCallback) {
     this.onEndCallback = cb
   }