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

FEATURES.md

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+# Features - Reader Android App
+
+Trang thai tinh nang mobile `reader-app` theo parity voi web.
+
+## Guest/User-facing
+
+| Feature | Status | Notes |
+|---|---|---|
+| Google login | done | Qua `/api/auth/mobile-login` |
+| Home/browse boards | done | `/api/novels/browse` |
+| Genre list | done | `/api/genres` |
+| Novel detail + chapter list | done | `/api/novels/{idOrSlug}`, `/api/truyen/{id}/chapters` |
+| Reader chapter detail | done | `/api/chapters/{chapterId}` |
+| Bookmark | done | `/api/user/bookmarks` |
+| Reading progress sync | done | `/api/user/reading-progress` |
+| Comments | done | `/api/truyen/{id}/comments` |
+
+## Parity Gaps
+
+| Feature | Status | Notes |
+|---|---|---|
+| User settings sync | planned | `/api/user/settings` |
+| User recommendations | planned | `/api/user/recommendations` |
+| Rating | planned | `/api/truyen/{id}/rate` |
+| Search suggest | planned | `/api/truyen/suggest` |
+
+## Dependencies
+
+- Contract: `reader-app/CONTRACT.md`
+- Mapping: `reader-app/CROSS_REPO_ENDPOINT_MATRIX.md`