fix(android): 升级 Kotlin 版本至 2.1.0

按 Flutter 建议升级 Kotlin 版本以避免兼容性问题

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

frontend/mobile-app/android/settings.gradle.kts

@@ -20,7 +20,7 @@ pluginManagement {
 plugins {
     id("dev.flutter.flutter-plugin-loader") version "1.0.0"
     id("com.android.application") version "8.11.1" apply false
-    id("org.jetbrains.kotlin.android") version "1.9.22" apply false
+    id("org.jetbrains.kotlin.android") version "2.1.0" apply false
 }
 
 include(":app")