feat: add version check app

docs/flutter-app-version-prompt.md

@@ -0,0 +1,34 @@
+## KoolTap Flutter App — Send the App Version, Ask to Update When It's Too Old
+
+The backoffice now refuses terminals running an app that is too old. Every call to `/devices/...` (register, sync,
+image upload, command ack) must carry the app version in a header:
+
+```
+X-App-Version: 1.4.2+37
+```
+
+Use the version from `pubspec.yaml` (read it at runtime with `package_info_plus`: `version` + `+` + `buildNumber`, or
+just `version`). The build number after `+` is ignored by the backoffice.
+
+When the app is older than the backoffice's minimum version, or sends no header (every app from before this change),
+the call fails **before** the token is checked with:
+
+```
+HTTP 426
+{ "error": { "code": "app_update_required", "message": "Update the app to version 1.4.0 or newer" } }
+```
+
+### What to Change
+
+1. **Send the header** on every request to the backoffice: add it where the HTTP client sets the `Authorization`
+   header (e.g. a shared client or interceptor in `lib/services/remote_sync_service.dart`), so register, sync, image
+   uploads and acks all carry it.
+2. **Handle 426 / `app_update_required`** in the sync loop and on the registration screen:
+   - Stop syncing (don't retry in a loop); keep the till working offline exactly as when the backoffice is unreachable,
+     and keep every outbox (tab events, stock outcomes, acks) so nothing is lost once the app is updated.
+   - Show a clear, persistent banner/dialog: "This app is out of date. Update it to keep syncing with the backoffice",
+     with the server's `message`.
+   - **Don't** treat it as an auth error: never unpair or throw away the device token.
+   - Retry after the app restarts (an updated app restarts anyway).
+3. Bump the app version in `pubspec.yaml` for the release that adds this, and tell the backoffice that version: it is
+   set as `MIN_APP_VERSION` (env var, or the constant in `apps/api/src/devices/app-version.ts`).