feat: 选择视频库后自动触发 AI 智能匹配 (#1256 前端部分)

- useStep2Materials 添加 useEffect，auto 模式下选择视频库自动调用 handleSmartMatch
- 触发条件：auto 模式 + 视频库已选 + 素材加载完成 + 素材数>0
- 离开 auto 模式时重置 autoTriggeredRef，确保模式切换后重新触发
- handleSmartMatch 由 useCallback 包装，引用稳定，安全加入依赖项
- 用户不再需要手动点击「让AI帮你选」按钮

Related to #1256

apps/web/src/pages/generate/hooks/useStep2Materials.ts

@@ -2,7 +2,7 @@
  * Step 2 素材选择 Hook
  * 组合素材库加载 + 智能匹配两个子 Hook
  */
-import { useCallback } from "react"
+import { useCallback, useEffect, useRef } from "react"
 import { formatDuration } from "../utils/formatDuration"
 import { useMaterialLibrary } from "./step2-materials/useMaterialLibrary"
 import { useSmartMatch } from "./step2-materials/useSmartMatch"
@@ -34,6 +34,25 @@ export function useStep2Materials({
     onSmartSelectedIdsChange,
   })
 
+  /* ── 自动触发智能匹配：选择视频库后自动调用 ── */
+  const autoTriggeredRef = useRef<string>("")
+  const { handleSmartMatch } = smartMatch
+  useEffect(() => {
+    // 离开 auto 模式时重置，确保下次进入 auto 模式能重新触发
+    if (materialMode !== "auto") {
+      autoTriggeredRef.current = ""
+      return
+    }
+    if (!selectedLibraryId) return
+    if (materialsLoading) return
+    if (materials.items.length === 0) return
+    // 防止同一视频库重复触发
+    if (autoTriggeredRef.current === selectedLibraryId) return
+
+    autoTriggeredRef.current = selectedLibraryId
+    handleSmartMatch()
+  }, [selectedLibraryId, materialMode, materialsLoading, materials.items, handleSmartMatch])
+
   /* ── 手动选择素材 ── */
   const handleToggleMaterial = useCallback(
     (materialId: string) => {