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/models.py

@@ -0,0 +1,29 @@
+from django.contrib.auth.models import AbstractUser
+from django.db import models
+
+
+class User(AbstractUser):
+    """Custom user model for Cloud Reader."""
+
+    email = models.EmailField(unique=True)
+    display_name = models.CharField(max_length=150, blank=True)
+    avatar = models.ImageField(upload_to="avatars/", blank=True, null=True)
+    is_verified = models.BooleanField(default=False)
+    reading_preferences = models.JSONField(default=dict, blank=True)
+
+    USERNAME_FIELD = "email"
+    REQUIRED_FIELDS = ["username"]
+
+    class Meta:
+        db_table = "accounts_user"
+        verbose_name = "User"
+        verbose_name_plural = "Users"
+
+    def __str__(self) -> str:
+        return self.email
+
+    @property
+    def avatar_url(self) -> str | None:
+        if self.avatar:
+            return self.avatar.url
+        return None