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

- Fix cookie path to '/' for auth persistence across page refreshes
- Login field now accepts both username and email
- Add 30s polling for group list and team session status refresh
- Add group name uniqueness check before creation

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

frontend/src/components/group/CreateGroupDialog.vue

@@ -4,6 +4,7 @@ import { useRouter } from 'vue-router'
 import { ElMessage } from 'element-plus'
 import { useGroupStore } from '@/stores/group'
 import { createGroup } from '@/api/groups'
+import pb from '@/api/pocketbase'
 
 const props = defineProps<{
   modelValue: boolean
@@ -33,7 +34,20 @@ async function handleSubmit() {
     ElMessage.warning('请输入群组名称')
     return
   }
+
+  // 检查群组名是否重复
   loading.value = true
+  try {
+    const existing = await pb.collection('groups').getList(1, 1, {
+      filter: `name="${form.value.name.trim()}"`
+    })
+    if (existing.items.length > 0) {
+      ElMessage.warning('该群组名称已存在，请换一个')
+      loading.value = false
+      return
+    }
+  } catch { /* ignore */ }
+
   try {
     const group = await createGroup({
       name: form.value.name.trim(),