Resolve GitHub Actions IPA Upload

My Flutter 3.24 app builds fine in GitHub Actions, but the workflow falls over when it tries to push the generated .ipa to App Store Connect. The job reports “incorrect file format,” so the upload step never completes.

I am working on Windows, so a fully automated macOS runner solution inside GitHub Actions is essential. The build stage already signs and produces the .ipa without errors; only the upload phase needs attention. You are free to introduce Fastlane or refine the existing scripts—whichever gets the artefact accepted by App Store Connect.

Deliverables
• Updated workflow YAML (and any auxiliary Fastlane files if you choose that route)
• Brief notes on required secrets and environment variables
• A successful test run that reaches “Processing in App Store Connect” without manual intervention

If you have solved this exact “incorrect file format” rejection before, I’d love to hear how you approach it.