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/config/django.py

@@ -41,6 +41,7 @@ INSTALLED_APPS = [
     "apps.books",
     "apps.annotations",
     "apps.reader",
+    "apps.groups",
 ]
 
 MIDDLEWARE = [

backend/config/urls.py

@@ -8,6 +8,7 @@ urlpatterns = [
     path("api/books/", include("apps.books.urls")),
     path("api/annotations/", include("apps.annotations.urls")),
     path("api/reader/", include("apps.reader.urls")),
+    path("api/", include("apps.groups.urls")),
 ]
 
 if settings.DEBUG: