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/QTESystem.ts

@@ -0,0 +1,75 @@
+import type { QTEDefinition } from '../types'
+
+type QTEUpdateCallback = (remaining: number, total: number) => void
+type QTEResultCallback = (success: boolean) => void
+
+export class QTESystem {
+  private timerId: ReturnType<typeof setInterval> | null = null
+  private timeoutId: ReturnType<typeof setTimeout> | null = null
+  private keyHandler: ((e: KeyboardEvent) => void) | null = null
+  private tickMs = 50
+  private active = false
+
+  trigger(
+    qte: QTEDefinition,
+    onUpdate: QTEUpdateCallback,
+    onResult: QTEResultCallback,
+  ) {
+    if (this.active) return
+    this.active = true
+
+    const startTime = Date.now()
+    const total = qte.timeLimit * 1000
+
+    this.keyHandler = (e: KeyboardEvent) => {
+      if (!this.active) return
+      const matched = qte.keys.some(
+        (k) => k.toLowerCase() === e.key.toLowerCase()
+      )
+      if (matched) {
+        this.clear()
+        onResult(true)
+      }
+    }
+    document.addEventListener('keydown', this.keyHandler)
+
+    this.timerId = setInterval(() => {
+      const elapsed = Date.now() - startTime
+      const remaining = Math.max(0, total - elapsed)
+      onUpdate(remaining / 1000, qte.timeLimit)
+      if (remaining <= 0) {
+        this.clear()
+        onResult(false)
+      }
+    }, this.tickMs)
+
+    this.timeoutId = setTimeout(() => {
+      this.clear()
+      onResult(false)
+    }, total)
+  }
+
+  cancel() {
+    this.clear()
+  }
+
+  private clear() {
+    this.active = false
+    if (this.timerId !== null) {
+      clearInterval(this.timerId)
+      this.timerId = null
+    }
+    if (this.timeoutId !== null) {
+      clearTimeout(this.timeoutId)
+      this.timeoutId = null
+    }
+    if (this.keyHandler !== null) {
+      document.removeEventListener('keydown', this.keyHandler)
+      this.keyHandler = null
+    }
+  }
+
+  destroy() {
+    this.clear()
+  }
+}