build(config): delete orphaned assets instead of narrowing the manifest

#16 replaced the `assets/` wildcard with three explicit paths. That is correct
today but makes the failure mode for a forgotten registration an `Unable to load
asset` exception at runtime rather than at build time, which neither
`flutter analyze` nor the build would catch.

Restore the wildcard and delete the three files it was working around instead.
All are unreferenced in tracked source: `logo.png` has no callers and no
`flutter_launcher_icons` config, while `notification_icon.png` and
`notification_banner.png` are stale copies of the real Android drawables in
`android/app/src/main/res/`, which the notification reaches via
`R.drawable.notification_status_icon` and never through the Flutter bundle.

Same APK-size win, no hand-maintained list, and the dead files leave the repo.

Co-Authored-By: Claude Opus 5 <[email protected]>
Claude-Session: https://claude.ai/code/session_019HT2x2JvJNg7XNsJubfHFf

pubspec.yaml

@@ -51,9 +51,7 @@ flutter:
 
   # To add assets to your application, add an assets section, like this:
   assets:
-    - assets/background.png
-    - assets/brand.png
-    - assets/finland_flag_256.png
+    - assets/
 
   # An image asset can refer to one or more resolution-specific "variants", see
   # https://flutter.dev/assets-and-images/#resolution-aware.