fix: login persistence, username login, realtime refresh, group name uniqueness

- Remove loadFromCookie that overwrites valid localStorage auth data
- Set user status to idle on first login (was empty string)
- Default empty status to idle instead of away

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

frontend/src/api/pocketbase.ts

@@ -5,13 +5,7 @@ const pbUrl = import.meta.env.VITE_PB_URL || window.location.origin
 
 export const pb = new PocketBase(pbUrl)
 
-// 认证状态持久化
-pb.authStore.loadFromCookie(document.cookie)
-
-// 保存认证状态到 cookie
-pb.authStore.onChange(() => {
-  document.cookie = pb.authStore.exportToCookie({ httpOnly: false, path: '/' })
-})
+// SDK v0.21+ 自动使用 localStorage 持久化，无需手动 cookie 操作
 
 // 获取当前用户
 export function getCurrentUser() {