feat: onboarding optimization, invite links, admin roles, and event board

- Home: hide duplicate create/join buttons when user has no groups
- Invite links: /join/group/:id and /join/team/:id pages for one-click joining
- Admin: group admins field, ownership transfer, member management toggle
- Events: new events collection with RSVP (going/interested/maybe) and comments

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

frontend/src/views/Home.vue

@@ -78,7 +78,7 @@ function openGameDetail(game: Game) {
           <span v-if="userStore.user?.statusNote" class="status-note">{{ userStore.user.statusNote }}</span>
         </div>
       </div>
-      <div class="welcome-actions">
+      <div v-if="!hasNoGroup" class="welcome-actions">
         <button class="cta-btn cta-btn--primary" @click="showCreateGroup = true">
           <el-icon><Plus /></el-icon> 创建群组
         </button>