feat: 网盘设置加「默认账号」列, 同类型最多2个主账号

- 数据库 migration: cloud_configs 加 is_primary 字段
- 后端: togglePrimary API (PUT /admin/cloud-configs/:id/primary)
- 后端: getAndValidateCredential 优先选 is_primary 账号
- 前端: CloudConfig.vue 转存启用后加「默认账号」开关列
- 前端: api/index.ts 加 setPrimary 方法

packages/backend/src/routes/admin.routes.ts

@@ -4,7 +4,7 @@ import fs from "fs";
 import { execSync } from 'child_process';
 import { adminLimiter, loginLimiter } from '../middleware/rate-limit';
 import { getSaveRecords } from '../cloud/cloud.service';
-import { getCloudConfigs, getCloudConfigById, saveCloudConfig, deleteCloudConfig, getCloudConfigByType, testCloudConnection, testCloudConnectionWithCookie } from '../cloud/credential.service';
+import { getCloudConfigs, getCloudConfigById, saveCloudConfig, deleteCloudConfig, getCloudConfigByType, testCloudConnection, testCloudConnectionWithCookie, togglePrimary } from '../cloud/credential.service';
 // Note: check-in routes were removed (sign-in feature removed)
 import { getAllCloudTypes } from '../cloud/cloud-types.service';
 import { login, authMiddleware, verifyToken, changePassword } from '../admin/auth.service';
@@ -199,6 +199,20 @@ router.delete('/admin/cloud-configs/:id', (req: Request, res: Response) => {
   }
 });
 
+/**
+ * PUT /api/admin/cloud-configs/:id/primary — toggle primary status (max 2 per type)
+ */
+router.put('/admin/cloud-configs/:id/primary', (req: Request, res: Response) => {
+  try {
+    const id = parseInt(req.params.id as string);
+    const { primary } = req.body;
+    const config = togglePrimary(id, !!primary);
+    res.json(config);
+  } catch (err: any) {
+    res.status(400).json({ error: err.message || 'Failed to toggle primary status' });
+  }
+});
+
 /** POST /api/admin/cloud-configs/:type/test — test cloud connection (by type or id) */
 router.post('/admin/cloud-configs/:type/test', async (req: Request, res: Response) => {
   try {