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/reading/serializers.py

@@ -0,0 +1,35 @@
+from rest_framework import serializers
+
+from .models import Bookmark, Highlight, ReadingProgress
+
+
+class BookmarkSerializer(serializers.ModelSerializer[Bookmark]):
+    class Meta:
+        model = Bookmark
+        fields = ["id", "document", "page", "label", "created_at"]
+        read_only_fields = ["id", "created_at"]
+
+
+class HighlightSerializer(serializers.ModelSerializer[Highlight]):
+    class Meta:
+        model = Highlight
+        fields = ["id", "document", "page", "color", "text", "note", "created_at"]
+        read_only_fields = ["id", "created_at"]
+
+
+class ReadingProgressSerializer(serializers.ModelSerializer[ReadingProgress]):
+    class Meta:
+        model = ReadingProgress
+        fields = ["id", "document", "current_page", "total_pages", "percentage", "last_read_at"]
+        read_only_fields = ["id", "percentage", "last_read_at"]
+
+
+class ReadingProgressUpdateSerializer(serializers.ModelSerializer[ReadingProgress]):
+    class Meta:
+        model = ReadingProgress
+        fields = ["current_page", "total_pages"]
+
+    def validate_current_page(self, value: int) -> int:
+        if value < 1:
+            raise serializers.ValidationError("Page must be at least 1.")
+        return value