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

packages/shared/src/utils.ts

@@ -0,0 +1,117 @@
+/** Shared utility functions for Cloud Reader */
+
+/**
+ * Format an ISO date string to a human-readable date.
+ */
+export function formatDate(iso: string): string {
+  const date = new Date(iso);
+  return date.toLocaleDateString("en-US", {
+    year: "numeric",
+    month: "short",
+    day: "numeric",
+  });
+}
+
+/**
+ * Format a date as a relative time string (e.g., "2h ago", "3d ago").
+ */
+export function formatRelativeTime(iso: string): string {
+  const now = Date.now();
+  const then = new Date(iso).getTime();
+  const diffMs = now - then;
+  const seconds = Math.floor(diffMs / 1000);
+  const minutes = Math.floor(seconds / 60);
+  const hours = Math.floor(minutes / 60);
+  const days = Math.floor(hours / 24);
+
+  if (days > 0) return `${days}d ago`;
+  if (hours > 0) return `${hours}h ago`;
+  if (minutes > 0) return `${minutes}m ago`;
+  return "just now";
+}
+
+/**
+ * Validate an email address format.
+ */
+export function isValidEmail(email: string): boolean {
+  return /^[^\s@]+@[^\s@]+\.[^\s@]+$/.test(email);
+}
+
+/**
+ * Password strength check:
+ * - At least 8 characters
+ * - At least one uppercase letter
+ * - At least one lowercase letter
+ * - At least one digit
+ */
+export function isStrongPassword(password: string): boolean {
+  return (
+    password.length >= 8 &&
+    /[A-Z]/.test(password) &&
+    /[a-z]/.test(password) &&
+    /\d/.test(password)
+  );
+}
+
+/**
+ * Check if two passwords match.
+ */
+export function doPasswordsMatch(password: string, confirm: string): boolean {
+  return password === confirm;
+}
+
+/**
+ * Get a user-friendly reading status label.
+ */
+export function readingStatusLabel(status: string): string {
+  const labels: Record<string, string> = {
+    to_read: "To Read",
+    reading: "Reading",
+    finished: "Finished",
+    dnf: "Did Not Finish",
+  };
+  return labels[status] ?? status;
+}
+
+/**
+ * Format file size in bytes to a human-readable string.
+ */
+export function formatFileSize(bytes: number): string {
+  if (bytes < 1024) return `${bytes} B`;
+  if (bytes < 1024 * 1024) return `${(bytes / 1024).toFixed(1)} KB`;
+  return `${(bytes / (1024 * 1024)).toFixed(1)} MB`;
+}
+
+// ---- API endpoint constants ----
+
+export const API_ENDPOINTS = {
+  auth: {
+    login: "/api/auth/login/",
+    register: "/api/auth/register/",
+    tokenRefresh: "/api/auth/token/refresh/",
+    profile: "/api/auth/profile/",
+  },
+  books: {
+    list: "/api/books/",
+    detail: (id: number | string) => `/api/books/${id}/`,
+    genres: "/api/books/genres/",
+    authors: "/api/books/authors/",
+  },
+  ebooks: {
+    list: "/api/ebooks/",
+    detail: (id: number) => `/api/ebooks/${id}/`,
+    upload: "/api/ebooks/upload/",
+    toc: (id: number) => `/api/ebooks/${id}/toc/`,
+    content: (id: number, page: number) =>
+      `/api/ebooks/${id}/content/?page=${page}`,
+    progress: (id: number) => `/api/ebooks/${id}/progress/`,
+    settings: (id: number) => `/api/ebooks/${id}/settings/`,
+  },
+  annotations: {
+    list: "/api/annotations/",
+    bookmarks: "/api/annotations/bookmarks/",
+    notes: "/api/annotations/notes/",
+    bookmarkDetail: (id: string) => `/api/annotations/bookmarks/${id}/`,
+    noteDetail: (id: string) => `/api/annotations/notes/${id}/`,
+  },
+} as const;