Update tech stack to Flutter & Dart

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

README.md

@@ -6,7 +6,7 @@
 
 ```
 SnapLedger/
-├── android/     # Android 客户端 (Kotlin + Jetpack Compose)
+├── android/     # Android 客户端 (Flutter)
 ├── backend/     # 后端服务 (Python FastAPI)
 └── README.md
 ```
@@ -15,7 +15,7 @@ SnapLedger/
 
 | 层级 | 技术 |
 |------|------|
-| Android | Kotlin, Jetpack Compose |
+| Android | Flutter, Dart |
 | 后端 | Python, FastAPI |
 | 数据库 | PostgreSQL |
 | 对象存储 | MinIO (S3 兼容) |