fix: member status visibility, team creation improvements, join approval flow

- Fix other members' status not visible due to users collection viewRule restriction
- Fix empty status treated as 'away' instead of 'idle' in membersByStatus
- Auto-set creator to 'in_team' status when creating team session
- Filter current user from idle members invite list
- Fix group store isGroupOwner using pb.authStore instead of localStorage
- Add nginx no-cache headers for index.html
- Add join_requests collection migration and join approval flow
- Update groups collection rules and add requireApproval field
- Add Memory types for Phase 2 planning

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

backend/pb_migrations/1776480065_updated_users.js

@@ -0,0 +1,18 @@
+/// <reference path="../pb_data/types.d.ts" />
+migrate((db) => {
+  const dao = new Dao(db)
+  const collection = dao.findCollectionByNameOrId("_pb_users_auth_")
+
+  collection.listRule = "@request.auth.id != \"\""
+  collection.viewRule = "@request.auth.id != \"\""
+
+  return dao.saveCollection(collection)
+}, (db) => {
+  const dao = new Dao(db)
+  const collection = dao.findCollectionByNameOrId("_pb_users_auth_")
+
+  collection.listRule = "id = @request.auth.id"
+  collection.viewRule = "id = @request.auth.id"
+
+  return dao.saveCollection(collection)
+})