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/router/index.ts

@@ -83,6 +83,18 @@ const routes: RouteRecordRaw[] = [
       }
     ]
   },
+  {
+    path: '/join/group/:groupId',
+    name: 'JoinGroup',
+    component: () => import('@/views/JoinGroupPage.vue'),
+    props: true
+  },
+  {
+    path: '/join/team/:sessionId',
+    name: 'JoinTeam',
+    component: () => import('@/views/JoinTeamPage.vue'),
+    props: true
+  },
   {
     path: '/:pathMatch(.*)*',
     name: 'NotFound',