[codex] Enforce Android Firebase release config

[jjoonleo]

Summary

• Require Android release builds to include a Firebase google-services.json config instead of silently skipping Google Services.
• Add a manual Android release verification workflow that decodes ANDROID_GOOGLE_SERVICES_JSON_B64 and builds the release app bundle.
• Document the Android release config secret and keep generated source-set Firebase config files ignored.

Why

Issue #386 identified that release builds could compile without the Android Firebase client config, leaving Firebase Core and FCM behavior dependent on local/CI filesystem state.

Validation

• flutter analyze
• flutter test
• flutter build apk --debug without Android Firebase config
• flutter build appbundle --release without Android Firebase config, confirmed it fails with the intended Gradle error

Notes

Release success and real-device FCM registration still require the approved production google-services.json / ANDROID_GOOGLE_SERVICES_JSON_B64 secret.

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[github-actions]

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