Release signing: key.properties + keystore for make build-android

[valorengels]

Parent: #13 (Make the template a runnable Android project — missing Gradle scaffolding)

Problem

PR #17 restored the missing Android Gradle scaffolding so flutter build apk --debug succeeds from a clean checkout. The release build (make build-android / flutter build apk --release) is not verified by #17. Per the plan's No-Gos, real release signing is deferred to this concrete follow-up.

Current state (from #17):

• android/key.properties.example documents the storePassword / keyPassword / keyAlias / storeFile format.
• The generated android/app/build.gradle.kts still signs the release build with the debug keystore (Flutter default) and does not read key.properties.
• The example file carries a NOTE that release signing is tracked here.

Desired outcome

• make build-android (flutter build apk --release) succeeds from a clean checkout on a properly configured machine.
• android/app/build.gradle.kts reads key.properties (same format as key.properties.example) to sign the release build.
• key.properties and the release keystore stay gitignored (never committed).

Solution sketch

• Wire the release-signing block in android/app/build.gradle.kts to read key.properties (storePassword/keyPassword/keyAlias/storeFile), matching android/key.properties.example.
• Generate/create a release keystore and a real key.properties on the developer machine (kept out of git via the existing .gitignore entries).
• Verify flutter build apk --release succeeds.

Out of scope

• Committing any keystore or key.properties secret to the repo.