fix: enable core library desugaring for flutter_local_notifications

flutter_local_notifications requires isCoreLibraryDesugaringEnabled and
the desugar_jdk_libs dependency to build on API < 26.  Without this the
release AAB build fails with ':app:checkReleaseAarMetadata'.

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

android/app/build.gradle.kts

@@ -13,6 +13,7 @@ android {
     compileOptions {
         sourceCompatibility = JavaVersion.VERSION_17
         targetCompatibility = JavaVersion.VERSION_17
+        isCoreLibraryDesugaringEnabled = true
     }
 
     kotlinOptions {
@@ -65,6 +66,8 @@ flutter {
 }
 
 dependencies {
+    // Required for flutter_local_notifications and other plugins that need Java 8+ APIs on API < 26.
+    coreLibraryDesugaring("com.android.tools:desugar_jdk_libs:2.1.4")
     // integration_test is a dev dependency; the Flutter plugin loader adds it as
     // debugImplementation only, but GeneratedPluginRegistrant.java (in src/main)
     // references its class in all variants. Make it available for release compilation