feat: add local-first private AI digest workflow

Migrate app code into canonical feature slices, add phone-only AI digest scheduling and review, wire local notification/background task support, and cover the flow with tests.

test/features/app/app_auth_flow_test.dart

@@ -50,7 +50,7 @@ void main() {
       findsOneWidget,
     );
 
-    await tester.tap(find.widgetWithText(FilledButton, 'Sign Out'));
+    await tester.tap(find.widgetWithText(OutlinedButton, 'Sign Out'));
     await tester.pumpAndSettle(const Duration(milliseconds: 450));
 
     expect(find.text('Sign in to continue'), findsOneWidget);