Expand Flutter app scaffold

docs/push-platform-todo.md

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+# Push Platform TODO
+
+## Android
+
+- Firebase project and Android app registration
+- `google-services.json` handling policy
+- FCM permission and foreground/background/terminated delivery checks
+- Push token refresh handling
+
+## iOS
+
+- Apple bundle id, APNs key or certificate, and entitlement setup
+- Firebase iOS app registration if FCM is used as APNs gateway
+- Notification permission prompt timing
+- Foreground/background/terminated delivery checks
+
+## Repository Rules
+
+- Do not commit APNs keys, FCM server keys, signing keys, or production secrets.
+- Decide separately whether environment-specific Firebase config files are committed or injected in CI.