Project worktree defaults with fixed and last-used refs #13598

A project’s development branch can differ from its GitHub default branch. With new threads set to use worktrees, repeatedly choosing the same base is easy to forget. Some projects need a fixed base such as dev; others should reuse the last explicitly chosen base.

Add one Worktree base ref preference to the existing project settings, with three choices:

• Repository default, preserving the current default behavior.
• A specific branch, tag, or commit, with searchable project refs.
• Last used, remembering explicit base selections per project and environment on the current device.

Existing drafts and one-off branch choices should keep their selection. Automatic defaults should not overwrite the remembered choice. If the remembered branch no longer exists, fall back to the repository default, then the current checkout. Resetting a project override should restore the environment preference. Start from origin should continue to work independently.

Implementation: #13592, covering web/desktop and mobile. Related: the earlier, closed request #5279 and the last-used behavior proposed in #11991.