feat: support Chinese nickname in registration

- User input "昵称" stored in `name` field (supports Chinese)
- `username` auto-generated (PocketBase requires ASCII)
- Password rules displayed inline with real-time validation
- All UI displays prefer `name` over `username`

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

frontend/src/views/Layout.vue

@@ -11,6 +11,7 @@ import NotificationPanel from '@/components/common/NotificationPanel.vue'
 import CreateGroupDialog from '@/components/group/CreateGroupDialog.vue'
 import JoinGroupDialog from '@/components/group/JoinGroupDialog.vue'
 import { Monitor, HomeFilled, Grid, Plus, Search, Bell, AlarmClock, SwitchButton, Document } from '@element-plus/icons-vue'
+import { displayName } from '@/types'
 
 const router = useRouter()
 const route = useRoute()
@@ -151,7 +152,7 @@ const pageTitle = computed(() => {
             class="user-avatar"
             alt=""
           />
-          <span class="user-name">{{ userStore.user?.username }}</span>
+          <span class="user-name">{{ displayName(userStore.user) }}</span>
           <button class="logout-btn" @click="handleLogout" title="退出登录">
             <el-icon><SwitchButton /></el-icon>
           </button>