fix: 字体变更时重置fontLoaded状态并重新检测

- titleSettings变更时setFontLoaded(false)重置
- 使用document.fonts.check(fontSpec)精确检测目标字体
- 字体未加载时用document.fonts.load等待后重绘
- 移除setTimeout魔法数字，改用requestAnimationFrame

apps/web/src/pages/generate/components/PreviewVideoPanel.tsx

@@ -267,21 +267,50 @@ export const PreviewVideoPanel: React.FC<PreviewVideoPanelProps> = ({
     return () => observer.disconnect()
   }, [showTitlePreview, hasPreview, isError, drawNoVideoTitle])
 
-  // 字体加载完成后重绘（Canvas measureText 依赖字体已加载）
+  // 字体加载检测：当字体变更时重新检测，确保 measureText 使用正确字体计算
   useEffect(() => {
-    if (!showTitlePreview) return
+    if (!showTitlePreview || !titleSettings) {
+      setFontLoaded(false)
+      return
+    }
     let cancelled = false
-    document.fonts.ready.then(() => {
-      if (cancelled) return
-      // 短暂延迟确保字体完全可用
-      setTimeout(() => {
-        if (!cancelled) setFontLoaded(true)
-      }, 50)
-    })
+    setFontLoaded(false) // 字体变更时重置，防止使用 fallback 字体绘制
+
+    const fontWeight = titleSettings.bold ? "bold" : ""
+    const fontStyle = titleSettings.italic ? "italic" : ""
+    const fontSpec =
+      `${fontStyle} ${fontWeight} ${titleSettings.size}px "${titleSettings.font}"`.trim()
+
+    // 优先用 document.fonts.check 精确检测目标字体
+    if (document.fonts.check(fontSpec)) {
+      setFontLoaded(true)
+      return
+    }
+
+    // 字体尚未加载，等待后重试
+    document.fonts
+      .load(fontSpec)
+      .then(() => {
+        if (!cancelled) {
+          // requestAnimationFrame 确保在下一帧绘制前字体已可用
+          requestAnimationFrame(() => {
+            if (!cancelled) setFontLoaded(true)
+          })
+        }
+      })
+      .catch(() => {
+        // load 失败时回退到 fonts.ready
+        if (!cancelled) {
+          document.fonts.ready.then(() => {
+            if (!cancelled) setFontLoaded(true)
+          })
+        }
+      })
+
     return () => {
       cancelled = true
     }
-  }, [showTitlePreview])
+  }, [showTitlePreview, titleSettings])
 
   // video 加载完成后重绘
   const handleVideoLoaded = useCallback(() => {