Release Panelize 1.2.6

[Manho]

Summary

• bump Panelize metadata from 1.1.1 to 1.2.6
• add the 1.2.6 changelog entry
• prepare the release branch for the guarded publish workflow

Release changes

• fix update checks for GitHub-installed extensions by using GitHub Releases
• require the release branch to be merged into main before publishing

Validation

• Release Prep workflow completed successfully
• 316 unit tests passed in GitHub Actions
• release and Chrome Web Store ZIP artifacts were generated
• both ZIP files contain manifest.json version 1.2.6

After this PR is merged, run the manual Release Publish workflow with version 1.2.6 and this PR number.