feat(flutter): publish Android app to Google Play Store

[kauereinbold]

Goal

Ship the Flutter app to the Google Play Store once the core screens land (#337, #338, #339).

Scope

• Release signing: keystore + key.properties (gitignored), signed appBundle build
• App identity: final application id, icon, splash, store listing assets
• Play Console setup: internal testing track first, then production
• CI: release build workflow triggered by version tags, aligned with repo versioning rules
• Versioning: map pubspec.yaml version/build number to git tags

Acceptance

• flutter build appbundle --release produces a signed bundle
• App installable from Play internal testing track
• Release process documented in the app README

Parent: #325