fix: support initial winget publication

[jmanuelcorral]

Summary

• Updates the local WinGet manifest for jmanuelcorral.SquadCenter to v0.3.1 with the published installer SHA256.
• Fixes the manifest installer type to nullsoft, which passes winget validate for the NSIS installer.
• Updates the release workflow to detect whether the package already exists in microsoft/winget-pkgs and use wingetcreate submit for the first publication or wingetcreate update for later releases.
• Documents that WinGet availability depends on Microsoft accepting the initial package submission.

Validation

• winget validate --manifest .\winget\jmanuelcorral.SquadCenter.yaml --disable-interactivity
• python -c "import yaml, pathlib; yaml.safe_load(pathlib.Path('.github/workflows/release.yml').read_text())"
• git diff --check

Publishing note

Manual initial submission was attempted, but GitHub blocked access to microsoft/winget-pkgs because the token has not been authorized for Microsoft SAML SSO. The remaining external step is to rotate/authorize WINGET_TOKEN with a GitHub PAT that expires in 90 days or less and has SSO authorization for microsoft/winget-pkgs.