fix(config): silently skip readonly fields on save instead of rejecting

Frontend sends entire form state including readonly fields (PORT,
WEBHOOK_SECRET, JWT_SECRET). Previously the backend rejected the whole
request. Now readonly fields are silently skipped.

Ultraworked with [Sisyphus](https://github.com/code-yeongyu/oh-my-opencode)

src/controllers/config.ts

@@ -231,9 +231,8 @@ configRouter.put('/', async (c) => {
         errors.push(`未知配置项: ${key}`);
         continue;
       }
-      // Reject readonly fields
+      // Silently skip readonly fields — frontend sends entire form state
       if (field.readonly) {
-        errors.push(`${field.label}（${key}）为只读配置，无法通过 GUI 修改`);
         continue;
       }