Configure permanent Android release signing

[SamDozer]

Release builds currently fall back to the debug keystore when
apps/guitar_practice_app/android/key.properties is absent. The build says so
out loud, and the resulting APK installs fine — but a debug-signed artifact is
not publishable.

Why it matters now: the signing key chosen for the first real release is
permanent. Every future update must be signed with the same key, or existing
installs cannot upgrade.

• Generate a release keystore and back it up somewhere off this repository
• Create key.properties locally (already gitignored)
• Rebuild the split APKs and the AAB
• Confirm the APK installs over an existing install, or accept that beta
  testers must uninstall once
• Decide between keeping the key locally and Play App Signing

Never commit the keystore, the passwords, or key.properties.