feat: 新增自定义配置合并到 Surge 输出

管理面板新增「自定义」面板，可按 Surge 格式书写 [Proxy]/[Rule]/
[General] 等配置片段，生成时合并进最终输出：[Proxy] 行追加到节点
之后，[Rule] 行注入到规则最顶部（优先级最高），其他段注入对应段
顶部且同名 key 覆盖模板取值（如 doh-server）。仅管理员链接生效，
游客链接不包含自定义内容。

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

server/src/routes/surge.ts

@@ -18,6 +18,22 @@ router.post('/surge-token', (_req, res) => {
   res.json({ token });
 });
 
+// GET /api/config/custom - get custom config snippet
+router.get('/custom', (_req, res) => {
+  const row = db.prepare("SELECT value FROM config WHERE key = 'custom_config'").get() as any;
+  res.json({ content: row?.value || '' });
+});
+
+// PUT /api/config/custom - save custom config snippet
+router.put('/custom', (req, res) => {
+  const { content } = req.body;
+  if (typeof content !== 'string') {
+    return res.status(400).json({ error: 'content must be a string' });
+  }
+  db.prepare("INSERT OR REPLACE INTO config (key, value) VALUES ('custom_config', ?)").run(content);
+  res.json({ ok: true });
+});
+
 // GET /api/config/preview - preview generated config
 router.get('/preview', (req, res) => {
   const host = req.headers.host || 'localhost:3456';