fix(settings): opens GitHub app settings for org access

[wgordon17]

Summary

• Replaces broken re-auth OAuth redirect with GitHub's app connection settings page where users can actually manage org access
• Auto-merges newly accessible orgs on window focus when user returns, with "Syncing..." disabled state and warning notification on failure
• Adds clientId format validation, isSafeGitHubUrl guard before registering focus listener, and removes unused MERGE_ORGS_KEY