feat: implement mobile reader

- implement mobile epub reader

mobile/src/api/client.ts

@@ -1,5 +1,6 @@
 import axios, { type AxiosError, type InternalAxiosRequestConfig } from "axios";
 import AsyncStorage from "@react-native-async-storage/async-storage";
+import type { TokenResponse } from "@cloud-reader/shared";
 
 const STORAGE_KEYS = {
   ACCESS_TOKEN: "access_token",
@@ -39,6 +40,26 @@ async function clearTokens(): Promise<void> {
   ]);
 }
 
+async function saveTokens(tokens: TokenResponse): Promise<void> {
+  await setTokens(tokens.access, tokens.refresh);
+}
+
+async function loadTokens(): Promise<TokenResponse | null> {
+  const [access, refresh] = await Promise.all([
+    getAccessToken(),
+    getRefreshToken(),
+  ]);
+  if (!access || !refresh) {
+    return null;
+  }
+  return { access, refresh };
+}
+
+/** Absolute base URL used for direct (non-axios) requests like file downloads. */
+function getApiBaseUrl(): string {
+  return api.defaults.baseURL ?? "";
+}
+
 // ── Request interceptor ─────────────────────────────────────────────
 
 api.interceptors.request.use(async (config: InternalAxiosRequestConfig) => {
@@ -136,5 +157,14 @@ api.interceptors.response.use(
   },
 );
 
-export { getAccessToken, getRefreshToken, setTokens, clearTokens };
+const apiClient = api;
+
+export {
+  apiClient,
+  getAccessToken,
+  saveTokens,
+  loadTokens,
+  clearTokens,
+  getApiBaseUrl,
+};
 export default api;