Build Edit Contact Screen

Use a `BLoC` or `AsyncNotifier` (Riverpod) to manage form submission state: `idle | submitting | success | error`. Drive the save button enabled/disabled state and loading indicator from this state. Implement dirty-state tracking by comparing the current form field values against the original contact snapshot captured on mount. For the unsaved-changes dialog, use a `PopScope` widget (Flutter 3.16+) with `onPopInvokedWithResult` to intercept back navigation.

GoRouter redirect guard should be implemented in the route's `redirect` callback, not inside the screen widget. Pass `autofocus: true` to the first field with an error after a failed submit to improve keyboard navigation flow.

Widget tests (flutter_test): (1) verify screen pre-populates fields from mock contact data; (2) submit with invalid data — verify inline field errors appear and form is not submitted; (3) simulate successful submit — verify `SnackBar` shown and navigator pops; (4) simulate network error — verify error banner appears and form state is preserved; (5) verify save button is disabled during in-progress state; (6) verify unsaved-changes dialog appears on back press when form is dirty; (7) verify read-only role is redirected. Accessibility: use `SemanticsHandle` to confirm all inputs have labels and error messages are announced. Target ≥85% widget test coverage.