When the base is an origin/… ref, fetch just that branch so the
local tracking ref is fresh before `git worktree add -b new origin/main`.
Fetch failures (offline / no remote) are silently ignored — git uses
whatever local ref exists, or raises a clear error if it's missing.
The sidebar "New worktree" button branched off whatever HEAD you were
on — now a filterable Popover+Command combobox lets you pick any local
or remote-tracking branch as the base, defaulting to origin/HEAD.
Backend listBaseBranches() queries refs/heads + refs/remotes via
for-each-ref, flags origin/HEAD (falling back to the local default for
no-remote repos). Mirrored on the Python REST API side
(base_branch_list + /api/git/base-branches route).
