feat: Integrate Expo mobile application into monorepo (#16)

- Add mobile/ directory with Expo React Native project
- Create API client with JWT auth and token refresh using AsyncStorage
- Implement AuthContext for login/register/logout flow
- Add screens: Login, Register, Library, Search, Settings
- Set up React Navigation with AuthStack and MainTabs
- Create packages/shared/ with shared types and utilities
- Add shared validation utilities (email, password strength)
- Update root package.json workspaces to include mobile + shared
- Add spec document docs/backend/009-expo-integration.md

mobile/src/api/books.ts

@@ -0,0 +1,31 @@
+import api from "./client";
+import type { Book, PaginatedResponse } from "@cloud-reader/shared";
+
+export function fetchBooks(
+  page = 1,
+  pageSize = 20,
+): Promise<PaginatedResponse<Book>> {
+  return api
+    .get<PaginatedResponse<Book>>("/api/books/", {
+      params: { page, page_size: pageSize },
+    })
+    .then((res) => res.data);
+}
+
+export function fetchBook(id: string): Promise<Book> {
+  return api.get<Book>(`/api/books/${id}/`).then((res) => res.data);
+}
+
+export function searchBooks(
+  query: string,
+): Promise<PaginatedResponse<Book>> {
+  return api
+    .get<PaginatedResponse<Book>>("/api/books/search/", {
+      params: { q: query },
+    })
+    .then((res) => res.data);
+}
+
+export function deleteBook(id: string): Promise<void> {
+  return api.delete(`/api/books/${id}/`).then(() => {});
+}