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/accounts/serializers.py

@@ -0,0 +1,53 @@
+from django.contrib.auth import get_user_model
+from rest_framework import serializers
+
+from .models import User
+
+UserModel = get_user_model()
+
+
+class RegisterSerializer(serializers.ModelSerializer[User]):
+    password = serializers.CharField(write_only=True, min_length=8)
+    password_confirm = serializers.CharField(write_only=True, min_length=8)
+
+    class Meta:
+        model = User
+        fields = ["email", "username", "display_name", "password", "password_confirm"]
+
+    def validate(self, attrs):
+        if attrs["password"] != attrs.pop("password_confirm"):
+            raise serializers.ValidationError({"password_confirm": "Passwords do not match."})
+        return attrs
+
+    def create(self, validated_data):
+        password = validated_data.pop("password")
+        user = UserModel(**validated_data)
+        user.set_password(password)
+        user.save()
+        return user
+
+
+class UserSerializer(serializers.ModelSerializer[User]):
+    avatar_url = serializers.SerializerMethodField()
+
+    class Meta:
+        model = User
+        fields = [
+            "id", "email", "username", "display_name", "avatar_url",
+            "date_joined", "is_verified", "reading_preferences",
+        ]
+        read_only_fields = ["id", "email", "date_joined", "is_verified"]
+
+    def get_avatar_url(self, obj: User) -> str | None:
+        return obj.avatar_url
+
+
+class ChangePasswordSerializer(serializers.Serializer):
+    old_password = serializers.CharField(required=True)
+    new_password = serializers.CharField(required=True, min_length=8)
+
+    def validate_old_password(self, value: str) -> str:
+        user = self.context["request"].user
+        if not user.check_password(value):
+            raise serializers.ValidationError("Current password is incorrect.")
+        return value