build(android): 压缩生产包体积至 13MB 并统一双渠道产物

生产包九成体积是两套 ABI 的引擎与快照。release 固定 split-debug-info 与
dart-obfuscation，快照从 8MB 降到 1.1MB/ABI；原生库改为压缩存储；开启
R8 裁剪并补保号规则；只打包实际引用的登录背景，移除零引用的图标字体。

pubspec.yaml

@@ -33,9 +33,6 @@ dependencies:
   flutter:
     sdk: flutter
 
-  # The following adds the Cupertino Icons font to your application.
-  # Use with the CupertinoIcons class for iOS style icons.
-  cupertino_icons: ^1.0.8
   # 网络层用 dio（统一拦截器/超时/错误映射）；纯 Dart 无原生插件，仍然不引代码生成与图片缓存库
   dio: ^5.7.0
   provider: ^6.1.5+1
@@ -90,7 +87,8 @@ flutter:
     - assets/icon/splash_logo.png
     - assets/icon/splash_logo_android12.png
     - assets/icon/rail/
-    - assets/auth/
+    # 只打包代码实际引用的那张登录背景（其余 3 张留在仓库但不进包）。
+    - assets/auth/bg_3.jpg
 
   # To add assets to your application, add an assets section, like this:
   # assets: