fix: 修复Gin通配符路由冲突导致panic

- 改用NoRoute替代通配符路由/*filepath
- NoRoute中先检查静态文件存在再返回SHTML

backend/cmd/main.go

@@ -261,24 +261,20 @@ func setupEmbeddedFrontend(r *gin.Engine) {
 		return
 	}
 
-	fileServer := http.FileServer(http.FS(distFS))
-
 	r.GET("/assets/*filepath", func(c *gin.Context) {
 		c.FileFromFS(c.Request.URL.Path, http.FS(distFS))
 	})
 
-	r.GET("/*filepath", func(c *gin.Context) {
-		path := c.Param("filepath")
-		if path == "" || path == "/" {
-			serveIndexHTML(c, string(indexHTML))
+	r.NoRoute(func(c *gin.Context) {
+		if c.Request.Method != "GET" || strings.HasPrefix(c.Request.URL.Path, "/api/") {
+			c.JSON(404, gin.H{"error": "not found"})
 			return
 		}
-		path = strings.TrimPrefix(path, "/")
 
-		f, err := distFS.Open(path)
-		if err == nil {
+		path := strings.TrimPrefix(c.Request.URL.Path, "/")
+		if f, err := distFS.Open(path); err == nil {
 			f.Close()
-			fileServer.ServeHTTP(c.Writer, c.Request)
+			c.FileFromFS(c.Request.URL.Path, http.FS(distFS))
 			return
 		}