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

src/App.vue

@@ -2,6 +2,8 @@
 import { ref } from 'vue'
 import GamePlayer from '@/components/GamePlayer.vue'
 import ChoicePanel from '@/components/ChoicePanel.vue'
+import QTEOverlay from '@/components/QTEOverlay.vue'
+import Subtitles from '@/components/Subtitles.vue'
 import SaveLoadMenu from '@/components/SaveLoadMenu.vue'
 import { useGameEngine } from '@/composables/useGameEngine'
 import { useGameStore } from '@/stores/gameStore'
@@ -67,7 +69,20 @@ init()
     <template v-else>
       <div class="game-screen">
         <GamePlayer @video-ready="onVideoReady" />
+        <Subtitles
+          :current-time="store.videoTime"
+          :video-url="store.currentScene?.subtitleUrl ?? null"
+        />
+        <QTEOverlay
+          :visible="store.qteActive"
+          :prompt="store.qteDef?.prompt ?? ''"
+          :keys="store.qteDef?.keys ?? []"
+          :total="store.qteTotal"
+          :remaining="store.qteRemaining"
+          :result="store.qteResult"
+        />
         <ChoicePanel
+          v-if="!store.qteActive"
           :choices="store.choices"
           :timer-total="store.timerTotal"
           :timer-remaining="store.timerRemaining"