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

public/scenes/demo.json

@@ -8,11 +8,11 @@
     "intro": {
       "id": "intro",
       "videoUrl": "/videos/intro.mp4",
+      "subtitleUrl": "/subtitles/intro.vtt",
       "choices": [
         {
           "text": "走向左边那扇发光的门",
           "targetScene": "left_door",
-          "timeLimit": 5,
           "effects": [
             { "type": "add", "target": "courage", "value": 10 }
           ]
@@ -33,6 +33,7 @@
     "left_door": {
       "id": "left_door",
       "videoUrl": "/videos/left_door.mp4",
+      "subtitleUrl": "/subtitles/left_door.vtt",
       "choices": [
         {
           "text": "与陌生人握手",
@@ -50,6 +51,36 @@
     "right_door": {
       "id": "right_door",
       "videoUrl": "/videos/right_door.mp4",
+      "qte": {
+        "triggerTime": 1.0,
+        "prompt": "躲避飞来的石块！",
+        "keys": ["ArrowLeft", "ArrowRight", "a", "d"],
+        "timeLimit": 3.0,
+        "successScene": "qte_success",
+        "failScene": "qte_fail",
+        "effects": {
+          "success": [{ "type": "add", "target": "courage", "value": 15 }],
+          "fail": [{ "type": "add", "target": "trust", "value": -20 }]
+        }
+      }
+    },
+    "qte_success": {
+      "id": "qte_success",
+      "videoUrl": "/videos/qte_success.mp4",
+      "choices": [
+        {
+          "text": "继续前进",
+          "targetScene": "continue_ending"
+        },
+        {
+          "text": "回头",
+          "targetScene": "intro"
+        }
+      ]
+    },
+    "qte_fail": {
+      "id": "qte_fail",
+      "videoUrl": "/videos/qte_fail.mp4",
       "choices": [
         {
           "text": "继续前进",
@@ -64,6 +95,7 @@
     "stay": {
       "id": "stay",
       "videoUrl": "/videos/stay.mp4",
+      "subtitleUrl": "/subtitles/stay.vtt",
       "nextScene": "alone_ending"
     },
     "trust_ending": {