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/stores/gameStore.ts

@@ -1,11 +1,12 @@
 import { defineStore } from 'pinia'
 import { ref, shallowRef } from 'vue'
-import type { SceneNode, Choice } from '@engine/types'
+import type { SceneNode, Choice, QTEDefinition } from '@engine/types'
 
 export interface SlotInfo {
   slot: number
   timestamp: number
   sceneLabel: string
+  thumbnail?: string
 }
 
 export const useGameStore = defineStore('game', () => {
@@ -16,6 +17,13 @@ export const useGameStore = defineStore('game', () => {
   const timerRemaining = ref(0)
   const saves = ref<SlotInfo[]>([])
 
+  const qteActive = ref(false)
+  const qteDef = shallowRef<QTEDefinition | null>(null)
+  const qteTotal = ref(0)
+  const qteRemaining = ref(0)
+  const qteResult = ref<'none' | 'success' | 'fail'>('none')
+  const videoTime = ref(0)
+
   function setScene(scene: SceneNode) {
     currentScene.value = scene
   }
@@ -46,9 +54,36 @@ export const useGameStore = defineStore('game', () => {
     saves.value = list
   }
 
+  function showQTE(qte: QTEDefinition) {
+    qteActive.value = true
+    qteDef.value = qte
+    qteTotal.value = qte.timeLimit
+    qteRemaining.value = qte.timeLimit
+    qteResult.value = 'none'
+  }
+
+  function updateQTE(remaining: number) {
+    qteRemaining.value = remaining
+  }
+
+  function resolveQTE(success: boolean) {
+    qteResult.value = success ? 'success' : 'fail'
+    setTimeout(() => {
+      qteActive.value = false
+      qteDef.value = null
+      qteResult.value = 'none'
+    }, 1000)
+  }
+
+  function setVideoTime(t: number) {
+    videoTime.value = t
+  }
+
   return {
     currentScene, choices, gameEnded, timerTotal, timerRemaining, saves,
+    qteActive, qteDef, qteTotal, qteRemaining, qteResult, videoTime,
     setScene, setChoices, clearChoices, setGameEnded,
     setTimer, clearTimer, setSaves,
+    showQTE, updateQTE, resolveQTE, setVideoTime,
   }
 })