fix: capture save thumbnail on videoEnd instead of sceneChange

Previously thumbnail was captured during sceneChange (before video starts
playing), so readyState was low and capture was skipped. Now capture on
videoEnd when frame is guaranteed visible, store in lastThumbnail, and
reuse for both auto-save (slot 0) and manual save.

src/composables/useGameEngine.ts

@@ -8,6 +8,7 @@ export function useGameEngine(videoEls: () => [HTMLVideoElement | null, HTMLVide
   const engine = new Engine()
   const saveSystem = new SaveSystem()
   const store = useGameStore()
+  let lastThumbnail: string | undefined
 
   if (import.meta.env.DEV) {
     ;(window as any).__sm = engine.stateManager
@@ -42,7 +43,21 @@ export function useGameEngine(videoEls: () => [HTMLVideoElement | null, HTMLVide
       store.clearTimer()
     })
 
-    engine.on('videoEnd', () => {})
+    engine.on('videoEnd', () => {
+      try {
+        const video = engine.videoManager.getActiveVideoElement()
+        if (video && video.readyState >= 2) {
+          const canvas = document.createElement('canvas')
+          canvas.width = 320
+          canvas.height = 180
+          const ctx = canvas.getContext('2d')
+          if (ctx) {
+            ctx.drawImage(video, 0, 0, 320, 180)
+            lastThumbnail = canvas.toDataURL('image/jpeg', 0.6)
+          }
+        }
+      } catch { /* ignore */ }
+    })
 
     engine.on('gameEnd', () => {
       store.setGameEnded(true)
@@ -92,29 +107,13 @@ export function useGameEngine(videoEls: () => [HTMLVideoElement | null, HTMLVide
     const state = engine.stateManager
     const currentScene = store.currentScene
 
-    // Capture thumbnail from active video
-    let thumbnail: string | undefined
-    try {
-      const video = engine.videoManager.getActiveVideoElement()
-      if (video && video.readyState >= 2) {
-        const canvas = document.createElement('canvas')
-        canvas.width = 320
-        canvas.height = 180
-        const ctx = canvas.getContext('2d')
-        if (ctx) {
-          ctx.drawImage(video, 0, 0, 320, 180)
-          thumbnail = canvas.toDataURL('image/jpeg', 0.6)
-        }
-      }
-    } catch { /* ignore canvas errors */ }
-
     await saveSystem.save(slot, {
       timestamp: Date.now(),
       currentScene: currentScene?.id ?? '',
       variables: state.variables,
       flags: [...state.flags],
       history: state.history,
-      thumbnail,
+      thumbnail: lastThumbnail,
     })
     await refreshSaves()
   }