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/ebooks.ts

@@ -0,0 +1,54 @@
+import { apiClient } from "./client";
+import type {
+  EBookListItem,
+  EBookDetail,
+  ReadingProgress,
+  ReadingSettings,
+  TocResponse,
+  ContentResponse,
+  PaginatedResponse,
+} from "@cloud-reader/shared";
+
+export const ebooksApi = {
+  /** List uploaded e-books */
+  list() {
+    return apiClient.get<PaginatedResponse<EBookListItem>>("/api/ebooks/");
+  },
+
+  /** Get e-book detail */
+  get(id: number) {
+    return apiClient.get<EBookDetail>(`/api/ebooks/${id}/`);
+  },
+
+  /** Get table of contents */
+  getToc(id: number) {
+    return apiClient.get<TocResponse>(`/api/ebooks/${id}/toc/`);
+  },
+
+  /** Get page content */
+  getContent(id: number, page: number) {
+    return apiClient.get<ContentResponse>(
+      `/api/ebooks/${id}/content/?page=${page}`,
+    );
+  },
+
+  /** Update reading progress */
+  updateProgress(id: number, data: Partial<ReadingProgress>) {
+    return apiClient.patch<ReadingProgress>(
+      `/api/ebooks/${id}/progress/`,
+      data,
+    );
+  },
+
+  /** Get or update reading settings */
+  getSettings(id: number) {
+    return apiClient.get<ReadingSettings>(`/api/ebooks/${id}/settings/`);
+  },
+
+  updateSettings(id: number, data: Partial<ReadingSettings>) {
+    return apiClient.patch<ReadingSettings>(
+      `/api/ebooks/${id}/settings/`,
+      data,
+    );
+  },
+};