fix: 删除 decodeSegment 的 startFrame 参数，修复关键帧错误

WebCodecs 解码器要求第一个 chunk 必须是关键帧。续解码从 startFrame=60
开始时，samplesCollected[60] 不是关键帧 → 报错 "A key frame is required"。

修复：
- 删除 startFrame 参数，sample 循环永远从 0 开始
- decodeAroundPosition 不再传 startFrame
- totalDecoded 计算不再使用 startFrame 偏移

apps/web/src/pages/generate/hooks/useCanvasPlayer.ts

@@ -479,12 +479,7 @@ export function useCanvasPlayer(
 
   // ── 初始化 VideoDecoder 并解码指定片段 ──
   const decodeSegment = useCallback(
-    async (
-      _buffer: ArrayBuffer,
-      meta: SegmentMeta,
-      maxFrames?: number,
-      startFrame: number = 0,
-    ): Promise<void> => {
+    async (_buffer: ArrayBuffer, meta: SegmentMeta, maxFrames?: number): Promise<void> => {
       console.log(`[DIAG_v2] >>> decodeSegment ENTER assetId=${meta.assetId}`)
       if (isDestroyedRef.current) return
 
@@ -580,7 +575,8 @@ export function useCanvasPlayer(
       let decodedCount = 0
       let skippedCount = 0
       let decodeErrors = 0
-      for (let si = startFrame; si < samplesCollected.length; si++) {
+      // 永远从 0 开始，确保第一个 sample 是关键帧
+      for (let si = 0; si < samplesCollected.length; si++) {
         const sample = samplesCollected[si]
         // seek/重播时旧的 decodeSegment 立即退出，不往帧队列推废帧
         if (decodeGenerationRef.current !== gen) {
@@ -651,7 +647,7 @@ export function useCanvasPlayer(
       )
 
       // 记录该片段已解码帧数（用于 decodeAroundPosition 续解码）
-      const totalDecoded = decodedCount + startFrame
+      const totalDecoded = decodedCount
       const metaArr = segmentMetaRef.current
       for (let idx = 0; idx < metaArr.length; idx++) {
         if (metaArr[idx].assetId === meta.assetId) {
@@ -719,7 +715,7 @@ export function useCanvasPlayer(
           const currentCount = decodedFrameCountsRef.current.get(i) ?? 0
           if (currentCount >= totalSamples) continue
           try {
-            await decodeSegment(buffer, meta, 300, currentCount)
+            await decodeSegment(buffer, meta, 300)
           } catch (e) {
             console.warn(`[useCanvasPlayer] 续解码片段 ${i} 失败:`, e)
           }
@@ -730,8 +726,7 @@ export function useCanvasPlayer(
         // 未开始解码的片段：开始解码
         decodedSegmentsRef.current.add(i)
         try {
-          const startFrame = decodedFrameCountsRef.current.get(i) ?? 0
-          await decodeSegment(buffer, meta, 300, startFrame)
+          await decodeSegment(buffer, meta, 300)
         } catch (e) {
           decodedSegmentsRef.current.delete(i)
           console.warn(`[useCanvasPlayer] 按需解码片段 ${i} 失败:`, e)