feat(app): checkout diff pane styling and e2e test support

- Style diff file cards edge-to-edge with single divider between files
- Remove monospace font from filenames for better readability
- Add checkout-ship e2e test for app
- Export Page type from e2e fixtures
- Add checkout query hooks for diff, status, and PR status

packages/app/e2e/fixtures.ts

@@ -100,4 +100,4 @@ test.afterEach(async ({ page }, testInfo) => {
   });
 });
 
-export { test, expect };
+export { test, expect, type Page };