v1.5.4+318: fix Android adaptive icon — replace stale 18T VN foreground with red pentagon to match Play Store listing

Google Play rejected v317 under the Misleading Claims policy because the AAB's adaptive icon foreground (used on Android 8+) was the pre-rebrand "18T VN" black/red grid design while the Play Store listing showed the current red pentagon icon. flutter_launcher_icons was driving from a stale assets/icon/app_icon.png; the pentagon mipmap legacy icons had been hand-replaced earlier but the adaptive foreground was never updated.

- assets/icon/app_icon.png: replace with red pentagon source
- assets/icon/app_icon_source.jpg: remove stale source
- pubspec.yaml: bump to +318, change adaptive_icon_background from #0A0A0A (black) to #B13131 (matches icon edge red so the OS-masked border blends)
- regenerate all Android mipmap legacy + drawable adaptive foreground PNGs and iOS AppIcon.appiconset variants from the new source via flutter_launcher_icons

Author: RedGridTactical