style: 修复 prettier 格式问题（useStep6Cover 长行换行）

- useCanvasPlayer: 初始化改为按需解码（前3片段+decodeAroundPosition动态加载）
- useCanvasPlayer: 新增 lastDrawnFrameRef 追踪，绘制新帧前关闭上一帧防止内存泄漏
- useCanvasPlayer: seek 时重置解码状态并重新解码当前区域
- useStep6Cover: 轮询增加 pollCount(max 60) + setTimeout 120s 双重超时保护
- useStep6Cover: 400错误精确匹配预览缺失模式，避免误触发自动修复

Closes: #1437, #1438, #1439

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

@@ -219,6 +219,8 @@ export function useCanvasPlayer(
   const isDestroyedRef = useRef(false)
   const lastProgressUpdateRef = useRef<number>(0)
   const descriptionCache = useRef<Map<string, ArrayBuffer>>(new Map())
+  const decodedSegmentsRef = useRef(new Set<number>())
+  const lastDrawnFrameRef = useRef<VideoFrame | null>(null)
 
   // 计算总时长
   const totalDuration = segments.reduce((sum, seg) => sum + (seg.endTime - seg.startTime), 0)
@@ -625,6 +627,43 @@ export function useCanvasPlayer(
     [],
   )
 
+  // ── 按需解码：根据当前播放位置解码附近片段 ──
+  const decodeAroundPosition = useCallback(
+    async (currentTime: number) => {
+      const metas = segmentMetaRef.current
+      if (metas.length === 0) return
+
+      // 找到当前时间对应的片段
+      let currentIdx = 0
+      for (let i = 0; i < metas.length; i++) {
+        if (currentTime >= metas[i].globalStartTime && currentTime < metas[i].globalEndTime) {
+          currentIdx = i
+          break
+        }
+        if (currentTime >= metas[metas.length - 1].globalEndTime) {
+          currentIdx = metas.length - 1
+        }
+      }
+
+      // 解码当前片段及前后各 1 个片段
+      const startIdx = Math.max(0, currentIdx - 1)
+      const endIdx = Math.min(metas.length - 1, currentIdx + 1)
+
+      for (let i = startIdx; i <= endIdx; i++) {
+        if (decodedSegmentsRef.current.has(i)) continue
+        if (isDestroyedRef.current) break
+
+        const meta = metas[i]
+        const buffer = segmentDataRef.current.get(meta.assetId)
+        if (!buffer) continue
+
+        decodedSegmentsRef.current.add(i)
+        await decodeSegment(buffer, meta)
+      }
+    },
+    [decodeSegment],
+  )
+
   // ── Canvas 渲染循环 ──
   const renderFrame = useCallback(() => {
     if (isDestroyedRef.current) return
@@ -642,6 +681,11 @@ export function useCanvasPlayer(
     ctx.clearRect(0, 0, canvas.width, canvas.height)
 
     if (frame) {
+      // 关闭上一帧，防止 VideoFrame 资源泄漏
+      if (lastDrawnFrameRef.current) {
+        lastDrawnFrameRef.current.close()
+      }
+      lastDrawnFrameRef.current = frame
       const rect = computeDrawRect(canvas.width, canvas.height)
       ctx.drawImage(frame, rect.dx, rect.dy, rect.dw, rect.dh)
     }
@@ -660,6 +704,9 @@ export function useCanvasPlayer(
         }
         return s
       })
+
+      // 按需解码后续片段（每 200ms 检查一次，避免阻塞渲染）
+      decodeAroundPosition(currentTime)
     }
 
     if (currentTime >= totalDuration) {
@@ -668,7 +715,7 @@ export function useCanvasPlayer(
     }
 
     rafRef.current = requestAnimationFrame(renderFrame)
-  }, [canvasRef, totalDuration, titleSettings, drawTitle, computeDrawRect])
+  }, [canvasRef, totalDuration, titleSettings, drawTitle, computeDrawRect, decodeAroundPosition])
 
   // ── 播放控制 ──
   const play = useCallback(async () => {
@@ -687,15 +734,22 @@ export function useCanvasPlayer(
   }, [])
 
   const seek = useCallback(
-    (time: number) => {
+    async (time: number) => {
       const clampedTime = Math.max(0, Math.min(time, totalDuration))
       setState((s) => ({ ...s, currentTime: clampedTime }))
       playStartOffsetRef.current = clampedTime
       playStartRef.current = performance.now()
-      // seek 后清空帧队列，等待新帧解码
+      // seek 后清空帧队列和已解码标记，触发重新解码
       frameQueueRef.current.clear()
+      if (lastDrawnFrameRef.current) {
+        lastDrawnFrameRef.current.close()
+        lastDrawnFrameRef.current = null
+      }
+      decodedSegmentsRef.current.clear()
+      // 立即解码 seek 位置附近的片段
+      await decodeAroundPosition(clampedTime)
     },
-    [totalDuration],
+    [totalDuration, decodeAroundPosition],
   )
 
   const destroy = useCallback(() => {
@@ -706,10 +760,15 @@ export function useCanvasPlayer(
       decoderRef.current.close()
     }
 
+    if (lastDrawnFrameRef.current) {
+      lastDrawnFrameRef.current.close()
+      lastDrawnFrameRef.current = null
+    }
     frameQueueRef.current.clear()
     segmentDataRef.current.clear()
     segmentMetaRef.current = []
     descriptionCache.current.clear()
+    decodedSegmentsRef.current.clear()
   }, [])
 
   // ── 预加载下一个片段的数据 ──
@@ -759,10 +818,14 @@ export function useCanvasPlayer(
         videoDimRef.current = { width: metas[0].videoWidth, height: metas[0].videoHeight }
       }
 
-      // 4. 依次解码每个片段
-      for (const meta of metas) {
+      // 4. 按需解码：初始只解码前 3 个片段（当前 + 前后各 1），避免环形缓冲区溢出导致黑屏
+      decodedSegmentsRef.current.clear()
+      const initialEnd = Math.min(metas.length, 3)
+      for (let i = 0; i < initialEnd; i++) {
+        const meta = metas[i]
         const buffer = segmentDataRef.current.get(meta.assetId)
         if (!buffer) continue
+        decodedSegmentsRef.current.add(i)
         await decodeSegment(buffer, meta)
         if (isDestroyedRef.current) break
       }

apps/web/src/pages/generate/hooks/useStep6Cover.ts

@@ -114,9 +114,22 @@ export function useStep6Cover({
       const anyErr = err as any
       const statusCode = anyErr?.response?.status
 
-      // 400 错误：后端缺少预览视频，自动创建后重试
-      if (statusCode === 400) {
-        console.log("[Step6] 后端返回 400，尝试自动创建预览渲染任务...")
+      // 400 错误：仅当确认为"预览缺失"类错误时才执行自动修复
+      // eslint-disable-next-line @typescript-eslint/no-explicit-any
+      const errCode = anyErr?.response?.data?.code as string | undefined
+      // eslint-disable-next-line @typescript-eslint/no-explicit-any
+      const errMsg = (anyErr?.response?.data?.message ||
+        anyErr?.response?.data?.detail ||
+        "") as string
+      const isPreviewMissing =
+        statusCode === 400 &&
+        (errCode?.includes("PREVIEW") ||
+          errCode?.includes("preview") ||
+          /预览.*(不存在|缺失|未找到|not found)/i.test(errMsg) ||
+          /preview.*(not found|missing|not exist)/i.test(errMsg))
+
+      if (isPreviewMissing) {
+        console.log("[Step6] 后端返回 400（预览缺失），尝试自动创建预览渲染任务...")
         message.info("正在准备预览视频，请稍候...")
         try {
           const previewResp = await createPreview({
@@ -124,9 +137,17 @@ export function useStep6Cover({
             asset_ids: assetIds,
             duration: duration || 30,
           })
-          // 轮询等待预览渲染完成
+          // 轮询等待预览渲染完成（最大 60 次 / 总超时 2 分钟）
           await new Promise<void>((resolve, reject) => {
+            let pollCount = 0
+            const maxPolls = 60
             const poll = setInterval(async () => {
+              pollCount++
+              if (pollCount > maxPolls) {
+                clearInterval(poll)
+                reject(new Error("预览生成超时，请稍后重试"))
+                return
+              }
               try {
                 const status = await getPreviewStatus(previewResp.task_id)
                 if (status.status === "completed") {
@@ -141,6 +162,11 @@ export function useStep6Cover({
                 reject(e)
               }
             }, 3000)
+            // 总超时保护：2 分钟后强制终止
+            setTimeout(() => {
+              clearInterval(poll)
+              reject(new Error("预览生成超时，请稍后重试"))
+            }, 120_000)
           })
           message.success("预览视频就绪，重新生成封面...")
           // 重试封面生成