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/subtitles/intro.vtt

@@ -0,0 +1,7 @@
+WEBVTT
+
+00:00.000 --> 00:02.000
+你醒来发现自己在一个陌生的房间
+
+00:02.500 --> 00:03.000
+前方有两扇门，你必须做出选择

public/subtitles/left_door.vtt

@@ -0,0 +1,7 @@
+WEBVTT
+
+00:00.000 --> 00:02.500
+你走进了发光的门，来到一个明亮的大厅
+
+00:02.500 --> 00:03.000
+一位陌生人向你伸出了手

public/subtitles/stay.vtt

@@ -0,0 +1,4 @@
+WEBVTT
+
+00:00.000 --> 00:03.000
+你选择留在原地，时间缓缓流逝...