feat: monorepo structure with Django backend, React frontend, and Expo mobile app

- Backend: Django 5 + DRF with accounts, documents, collections, and reading apps
  - Custom User model with email-based auth, JWT via SimpleJWT
  - Full CRUD viewsets with ModelSerializer + DRF routers
  - pytest, Ruff, drf-spectacular (OpenAPI), whitenoise
  - Dockerfile for production deployment

- Frontend: React 18 + TypeScript + Vite
  - Lazy-loaded routes with ProtectedRoute/PublicRoute guards
  - Auth context with useReducer, token refresh interceptor
  - Pages: Login, Register, Library, Document Detail, Reader, Collections, Settings
  - Dark theme, responsive grid layout, Vite proxy to Django backend

- Mobile: Expo SDK 51 + React Native + Expo Router
  - File-based routing with login, register, and library screens
  - AsyncStorage for token persistence, token refresh interceptor
  - Shared API types via @cloud-reader/shared workspace package

- Shared: TypeScript types (API responses, auth, documents, etc.)
- CI/CD: 3 independent GitHub Actions pipelines (backend, frontend, mobile)

backend/apps/collections/serializers.py

@@ -0,0 +1,33 @@
+from rest_framework import serializers
+
+from .models import Collection
+
+
+class CollectionSerializer(serializers.ModelSerializer[Collection]):
+    cover_url = serializers.SerializerMethodField()
+    document_count = serializers.IntegerField(read_only=True)
+
+    class Meta:
+        model = Collection
+        fields = [
+            "id", "name", "description", "cover_url", "document_count",
+            "is_public", "created_at", "updated_at",
+        ]
+        read_only_fields = ["id", "document_count", "created_at", "updated_at"]
+
+    def get_cover_url(self, obj: Collection) -> str | None:
+        return obj.cover_url
+
+
+class CollectionDetailSerializer(CollectionSerializer):
+    documents = serializers.PrimaryKeyRelatedField(many=True, read_only=True)
+
+    class Meta(CollectionSerializer.Meta):
+        fields = CollectionSerializer.Meta.fields + ["documents", "owner"]
+
+
+class CollectionDocumentActionSerializer(serializers.Serializer):
+    document_ids = serializers.ListField(
+        child=serializers.IntegerField(),
+        allow_empty=False,
+    )