When "new worktree" branches off a remote-tracking ref like origin/main,
`git worktree add -b <branch> <dir> origin/main` auto-sets upstream
tracking (branch → origin/main), producing `branch:origin/main` in branch
listings. The user wants a standalone local branch — like `git checkout
origin/main && git checkout -b branch` — not one silently wired to the
remote. Add `--no-track` when the base is an `origin/` ref. Local branch
bases are unaffected (they never triggered tracking).
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).
