Generate app store screenshots programmatically during the build instead of committing static assets.

Changes

• Integration test infrastructure: Added integration_test dependency and test driver to capture screenshots at runtime
• Screenshot generation: Created integration_test/screenshot_test.dart that renders demo UI at target dimensions:
  • Phone: 1140×2616 (2 screenshots)
  • Tablet: 2248×2480 (2 screenshots)
  • macOS: 1280×800 (2 screenshots)
• Build workflow: Added step to run integration tests, generate screenshots, package into SwiftControl.screenshots.zip, and attach to releases

Implementation

Screenshots are generated by rendering Flutter widgets at specific screen sizes during test execution:

// Set target dimensions
binding.window.physicalSizeTestValue = const Size(1140, 2616);

// Render demo UI
await tester.pumpWidget(MaterialApp(home: DemoScreen()));

// Capture screenshot
await binding.takeScreenshot('mob1');

The test driver intercepts screenshot callbacks and writes PNG files to build/screenshots/ which are then zipped and attached to the GitHub release.

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