feat: background download service + queue persistence (v1.6.1)

- Add foreground service for background downloads with wake lock
- Persist download queue to SharedPreferences for app restart recovery
- Fix share intent causing app restart (singleTask + onNewIntent)
- Fix back button clearing state during loading
- Upgrade Kotlin to 2.3.0 for share_plus 12.0.1 compatibility
- Add WAKE_LOCK permission for foreground service

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 "2.2.20" apply false
+    id("org.jetbrains.kotlin.android") version "2.3.0" apply false
 }
 
 include(":app")