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/components/group/GroupMembersPanel.vue

@@ -130,13 +130,13 @@ async function onJoinRequestResponded(requestId: string) {
       <div v-for="member in members" :key="member.id" class="member-row">
         <img :src="member.avatar || '/default-avatar.svg'" class="member-avatar" alt="" />
         <div class="member-info">
-          <span class="member-name">{{ member.username }}</span>
+          <span class="member-name">{{ member.name || member.username }}</span>
           <span v-if="member.id === group.owner" class="owner-badge">群主</span>
         </div>
         <button
           v-if="isOwner && member.id !== group.owner && member.id !== userStore.userId"
           class="remove-btn"
-          @click="removeMember(member.id, member.username)"
+          @click="removeMember(member.id, member.name || member.username)"
         >
           移除
         </button>