feat(rbac): F3 前端 RBAC 按钮级控制

后端:
- 新增 GET /user/permissions 端点返回当前用户权限码
- Login 响应附带 permissions 数组
- collectUserPermissionCodes 用 raw SQL JOIN 避免 GORM many2many 表名问题
- admin 用户返回 ['*'] 通配符

前端:
- usePermission hook (hasPermission/hasAnyPermission/hasAllPermissions)
- <HasPermission> 组件 (permission/anyOf/allOf + fallback)
- zustand store 新增 permissions 字段
- Sidebar 菜单递归过滤 (42 菜单项全部标注 permission)
- ServerManager: 添加/导入/批量删除按钮按权限显隐
- ServerTableRow: 终端/编辑/文件/删除按权限显隐
- SystemUserTab: 新建/导入/编辑/删除按钮按权限显隐
- MainLayout: 页面刷新自动拉取权限
- Login: 登录时存储 permissions

验证:
- admin: permissions=['*'] 通配符
- viewer: 7 个只读权限 (dashboard:view/server:view/...)
- viewer 无 server:create/delete
- 10/10 回归测试通过
- npm run build 零错误

frontend/src/components/auth/Login.tsx

@@ -85,8 +85,8 @@ export function Login() {
     setError('');
     setLoading(true);
     try {
-      const result = await userApi.login(username, password) as { user: any; token: string };
-      useUserStore.getState().login(result.user, result.token);
+      const result = await userApi.login(username, password) as { user: any; token: string; permissions?: string[] };
+      useUserStore.getState().login(result.user, result.token, result.permissions ?? null);
       localStorage.setItem('token', result.token);
       navigate('/', { replace: true });
     } catch (err: any) {