fix(webdav): align MetaPassKey behavior with FTP auth logic

For guest users, the WebDAV password input serves as the meta folder
password (consistent with FTP anonymous/guest handling). For authenticated
users, MetaPassKey is set to empty string since their login password is
not the meta folder password.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

Author: Lanfei

server/webdav.go

@@ -143,7 +143,11 @@ func WebDAVAuth(c *gin.Context) {
 		return
 	}
 	common.GinWithValue(c, conf.UserKey, user)
-	common.GinWithValue(c, conf.MetaPassKey, password)
+	if user.IsGuest() {
+		common.GinWithValue(c, conf.MetaPassKey, password)
+	} else {
+		common.GinWithValue(c, conf.MetaPassKey, "")
+	}
 	c.Next()
 }