Align webhook branch validation (#1781)

[Chris-C1108]

Aligns the webhook branch check with GitHub's current default branch naming (#1781).

Uses payload.repository.default_branch which is already utilized in controllers/user.js:1706 for the import page, falling back to 'master' if absent.

No behavioral change for repositories using master as default.

[Martii]

Near duplicate of #1840

It's initially master and if you want to write the routines for a one time migrate to main for individual users you are welcome to.... otherwise this is not going to be merged in it's current state. Alignment is currently with git not GitHub.

[Chris-C1108]

Thanks for pointing to #1840 and for the direction.

Understood: default stays master, with an explicit one-way opt-in/migration to main per user on their GitHub auth in /user/preferences, wired through the user model, webhook, and repoManager.

I am glad to take this on. I will keep working on this branch to implement the preference UI, model field, and sync wiring accordingly.

[Chris-C1108]

Implemented the one-way main branch migration routine as discussed:

• Model: Added ghBranch to userSchema with default 'master'.
• Preferences UI: Added GitHub Integration section on /user/preferences (visible only to accounts with GitHub auth). Displays current default branch and a one-way migration action to main. Once migrated, cannot be reverted back to master.
• Controller & Route: Added POST handler on /user/preferences strictly enforcing the one-way transition to main for GitHub-authed accounts only.
• Webhook: Branch validation checks against aUser.ghBranch || 'master', preserving existing master behavior for unmigrated accounts.
• RepoManager: loadSyncs and loadScripts dynamically fetch using (that.user && that.user.ghBranch) || 'master'.

Tested locally across migrated and unmigrated user flows. Both commits are signed. Ready for review whenever convenient.