fix(git): the branch switcher lists and checks out branches only the remote has

GET /api/lanes/:id/git/branches read only refs/heads, so a branch someone
else pushed never showed after a fetch or pull, and `switch` refused it
(ENOBRANCH). It now also returns `remotes`: fetched remote-tracking branches
with no local branch of that name (origin/HEAD left out), listed under
"On the remote"; switching to one runs `git switch -c <b> --track
refs/remotes/<remote>/<b>` with the remote ref looked up, never passed
through (bare name only when exactly one remote has it, else EAMBIGUOUS).
The bar also re-reads the branch list after a fetch or pull instead of
keeping the list it loaded first.

client/src/lib/api.ts

@@ -4320,6 +4320,8 @@ export const api = {
       request<{
         available: boolean;
         branches: string[];
+        /** Fetched remote branches with no local one yet (`origin/feature`); absent from an older server. */
+        remotes?: string[];
         current: string | null;
         worktrees: LaneWorktree[];
       }>(gitUrl(id, "/branches")),