feat(desktop): choose the comparison base branch in the Git changes panel

### Problem

The Changes panel in the Session workbar always compares against one branch the backend resolves for you (`origin/HEAD` → `origin/main` → `origin/master` → `main` → `master`), with no way to change it.

Stacked work breaks on this. When a Session's branch is based on another feature branch instead of the repository default, the panel lists every commit the stack inherits — hundreds of files that are not part of the change — and the added/deleted counts are meaningless.

### Desired outcome

The panel lets me pick the comparison base from the branches that actually exist in the repository, shows which branch it compares against next to the current branch, and remembers the choice for that Session. Repositories with many branches must stay usable, so the list needs to be searchable.
### Alternatives or workarounds

Run `git diff ...HEAD` in the terminal and ignore the panel; temporarily repoint the repository's default branch. Considered a global setting instead of a per-Session choice, but different Sessions work on different stacks.