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/config/exceptions.py

@@ -0,0 +1,13 @@
+"""Custom exception handler that returns consistent JSON error responses."""
+from rest_framework.views import exception_handler
+
+
+def custom_exception_handler(exc, context):
+    """Wrap DRF's default handler to always return {'detail': ..., 'code': ...}."""
+    response = exception_handler(exc, context)
+    if response is not None:
+        data = response.data
+        # Flatten validation errors into a consistent shape
+        if isinstance(data, dict) and "detail" not in data:
+            response.data = {"detail": "Validation error", "fields": data, "code": "validation_error"}
+    return response