fix: add required --flavor flag to Android build/run commands in README

[Copilot]

flutter run and flutter build apk/appbundle fail on Android because the app defines stable/unstable product flavors — Flutter requires --flavor when flavors are configured. The README's bare commands produced cryptic errors, leading to "does Android even work?" confusion.

Changes

• Installation / Nix sections — split flutter run into Android (--flavor stable) and non-Android variants
• Build section — added --flavor stable --release to flutter build apk and flutter build appbundle

# Before (fails silently on Android)
flutter run
flutter build apk

# After
flutter run --flavor stable         # Android
flutter run                         # iOS / Web / Desktop
flutter build apk --flavor stable --release
flutter build appbundle --flavor stable --release

Original prompt

So does android work or not? Is there any workaround?

---

💬 Send tasks to Copilot coding agent from Slack and Teams to turn conversations into code. Copilot posts an update in your thread when it's finished.