feat: implement group creation and management (US #28)

Backend:
- Add groups Django app with models: Group, GroupMember, GroupInvite, JoinRequest
- Create serializers with business rule validation
- Implement GroupViewSet with full CRUD + custom actions (members, invites, roles, leave, join requests)
- Add JoinGroupViewSet for invite-based joining flow
- Register app in Django config and URL routing

Frontend:
- Add shared types for groups to @cloud-reader/shared
- Create groups API client (groupsApi)
- Build GroupsListPage, GroupDetailPage (member mgmt, invites, role transfer)
- Build CreateGroupPage and JoinGroupPage
- Add lazy-loaded routes to App.tsx with ProtectedRoute
- Add navigation links to Library header

Ref: #28

backend/apps/groups/urls.py

@@ -0,0 +1,12 @@
+from django.urls import include, path
+from rest_framework.routers import DefaultRouter
+
+from apps.groups.views import GroupViewSet, JoinGroupViewSet
+
+router = DefaultRouter()
+router.register(r"groups", GroupViewSet, basename="group")
+router.register(r"join", JoinGroupViewSet, basename="join-group")
+
+urlpatterns = [
+    path("", include(router.urls)),
+]