feat: add Japanese language support, three-language subtitles for all demo scenes

public/demo/intro/intro_ja.vtt

@@ -0,0 +1,7 @@
+WEBVTT
+
+00:00.000 --> 00:02.000
+見知らぬ部屋で目覚めた
+
+00:02.500 --> 00:03.000
+目の前に二つのドアがある。選ばなければならない。

public/demo/investigation_site/investigation_en.vtt

@@ -0,0 +1,4 @@
+WEBVTT
+
+00:00.000 --> 00:05.000
+You step into a messy room, carefully looking around...

public/demo/investigation_site/investigation_ja.vtt

@@ -0,0 +1,4 @@
+WEBVTT
+
+00:00.000 --> 00:05.000
+散らかった部屋に入り、周りを注意深く観察する…

public/demo/left_door/left_door_ja.vtt

@@ -0,0 +1,7 @@
+WEBVTT
+
+00:00.000 --> 00:02.500
+光るドアを通り抜けると、明るい広間に出た
+
+00:02.500 --> 00:03.000
+見知らぬ人が手を差し伸べてきた

public/demo/stay/stay_ja.vtt

@@ -0,0 +1,4 @@
+WEBVTT
+
+00:00.000 --> 00:02.500
+その場に留まることを選んだ。時間がゆっくりと過ぎていく…

public/scenes/demo.json

@@ -67,7 +67,8 @@
       "subtitleUrl": "intro/intro.vtt",
       "subtitles": {
         "zh": "intro/intro.vtt",
-        "en": "intro/intro_en.vtt"
+        "en": "intro/intro_en.vtt",
+        "ja": "intro/intro_ja.vtt"
       },
       "bgmUrl": "shared/calm_bgm.mp3",
       "bgmVolume": 0.6,
@@ -108,6 +109,11 @@
       "videoUrl": "",
       "imageUrl": "investigation_site/investigation_scene.jpg",
       "subtitleUrl": "investigation_site/investigation.vtt",
+      "subtitles": {
+        "zh": "investigation_site/investigation.vtt",
+        "en": "investigation_site/investigation_en.vtt",
+        "ja": "investigation_site/investigation_ja.vtt"
+      },
       "hotspots": [
         {
           "id": "hs_desk",
@@ -175,6 +181,11 @@
       "id": "left_door",
       "videoUrl": "left_door/left_door.mp4",
       "subtitleUrl": "left_door/left_door.vtt",
+      "subtitles": {
+        "zh": "left_door/left_door.vtt",
+        "en": "left_door/left_door_en.vtt",
+        "ja": "left_door/left_door_ja.vtt"
+      },
       "choices": [
         {
           "text": "与陌生人握手",
@@ -261,6 +272,11 @@
       "id": "stay",
       "videoUrl": "stay/stay_loop.mp4",
       "subtitleUrl": "stay/stay.vtt",
+      "subtitles": {
+        "zh": "stay/stay.vtt",
+        "en": "stay/stay_en.vtt",
+        "ja": "stay/stay_ja.vtt"
+      },
       "bgmUrl": "shared/calm_bgm.mp3",
       "bgmVolume": 0.6,
       "videoMuted": true,