feat: Add architecture and contract documentation for reader-app, including API standards and cross-repo mapping

ARCHITECTURE.md

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+# Reader Suite Architecture (Android/Flutter)
+
+Tai lieu nay mo ta vai tro `reader-app` trong he sinh thai doc truyen gom web + mobile + backend.
+
+## Vai tro trong he thong
+
+- `reader-app` la mobile client cho end-user feature.
+- KHONG chua logic nghiep vu canonic; backend (`reader-api`) la noi quyet dinh rule.
+- Muc tieu: feature parity voi web user-facing, tru workflow MOD/ADMIN.
+
+## Kien truc app
+
+- `lib/core`: config, networking, storage, app-level services.
+- `lib/features`: module theo domain (auth, browse, novel-detail, reader, bookshelf...).
+- `lib/shared`: widget dung chung.
+
+## Nguyen tac dong bo voi web + backend
+
+- Dung chung endpoint contract voi web, khong tao API rieng cho mobile neu khong can thiet.
+- Dung chung semantic cho state:
+  - bookmark
+  - reading-progress
+  - recommendation
+  - user-settings
+- Auth mobile qua JWT; backend phai map cung identity voi web.
+
+## Environment va ket noi
+
+- Local API mac dinh: `http://10.0.2.2:8000` (Android emulator).
+- Script `scripts/flutter_run_with_env.sh` la cach chuan de chay local.
+- `BASE_URL`, `GOOGLE_SERVER_CLIENT_ID`, `GOOGLE_CLIENT_ID` duoc truyen qua `--dart-define`.
+
+## Definition of Done (Mobile)
+
+- API calls tuan thu contract `reader-api` (status code + error format).
+- UX/state nhat quan voi web cho luong user-facing.
+- Da test tren it nhat Android emulator + 1 moi truong khac (iOS simulator hoac device).
+- Cac huong dan env/auth trong README van dung sau thay doi.