fix(android): 启用 core library desugaring — 修复 flutter_local_notifications 构建失败

flutter_local_notifications 插件需要 Java 8+ API desugaring 支持。
在 build.gradle.kts 中启用 isCoreLibraryDesugaringEnabled 并添加
desugar_jdk_libs:2.1.4 依赖。

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

frontend/genex-mobile/android/app/build.gradle.kts

@@ -20,6 +20,7 @@ android {
     ndkVersion = flutter.ndkVersion
 
     compileOptions {
+        isCoreLibraryDesugaringEnabled = true
         sourceCompatibility = JavaVersion.VERSION_17
         targetCompatibility = JavaVersion.VERSION_17
     }
@@ -67,6 +68,10 @@ android {
     }
 }
 
+dependencies {
+    coreLibraryDesugaring("com.android.tools:desugar_jdk_libs:2.1.4")
+}
+
 flutter {
     source = "../.."
 }