Remove indigo background from app icon, use transparent

- Add app_icon_bg_transparent.png (1024x1024 fully transparent PNG)
  as the adaptive icon background layer
- Switch image_path (legacy icon) to app_icon_fg.png so pre-Android 8
  devices also get the transparent-background icon
- Regenerate all mipmap densities via flutter_launcher_icons

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

android/app/src/main/res/mipmap-anydpi-v26/ic_launcher.xml

@@ -1,6 +1,6 @@
 <?xml version="1.0" encoding="utf-8"?>
 <adaptive-icon xmlns:android="http://schemas.android.com/apk/res/android">
-  <background android:drawable="@color/ic_launcher_background"/>
+  <background android:drawable="@drawable/ic_launcher_background"/>
   <foreground>
       <inset
           android:drawable="@drawable/ic_launcher_foreground"

pubspec.yaml

@@ -33,8 +33,8 @@ flutter_launcher_icons:
   android: true
   ios: false
   remove_alpha_ios: false
-  image_path: "assets/icon/app_icon.png"
-  adaptive_icon_background: "#6366f1"
+  image_path: "assets/icon/app_icon_fg.png"
+  adaptive_icon_background: "assets/icon/app_icon_bg_transparent.png"
   adaptive_icon_foreground: "assets/icon/app_icon_fg.png"
   min_sdk_android: 21