[Desktop] Choose base branch for Changes panel diff and 'Create PR manually'

Environment

Claude desktop app (macOS), Code tab

Problem — the feature is effectively useless without this

The Changes panel always diffs against main, and its "Create PR manually" button always targets main, with no way to choose another base branch.

In our repos, almost no work goes straight into main — feature/fix branches go into dev first. Because the base can't be changed:

• the +N / -N diff count shown in the panel is meaningless (it's measured against the wrong branch and shows huge numbers unrelated to the session's actual change), and
• the "Create PR manually" button opens a PR against the wrong base, so I can't use it at all.

As it stands, both the diff display and the PR button are features I simply can't use.

Request

• Let the user pick the base branch for the Changes panel diff and the PR button, and/or
• configure a default base branch per repository (e.g. dev).