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.

server/routes/lanes.js

@@ -48,6 +48,7 @@ const {
   isGitRepo,
   isValidBranchName,
   listBranches,
+  listRemoteBranches,
   listWorktrees,
   resetWorktree,
   resolveBase,
@@ -1913,7 +1914,9 @@ router.get("/:id/git/tree", async (req, res) => {
 });
 
 /**
- * The lane's own local branches, for the branch switcher. Separate from
+ * The lane's own local branches, for the branch switcher, plus `remotes`: the
+ * fetched remote-tracking branches with no local branch yet (`origin/feature`
+ * — switching to one creates its tracking branch). Separate from
  * `GET /branches` (which takes a candidate repo PATH from the query, for a
  * lane that does not exist yet); this one resolves the repo from the lane row,
  * so no path ever comes from the request.
@@ -1925,9 +1928,10 @@ router.get("/:id/git/branches", async (req, res) => {
   try {
     const { branches, current } = await listBranches(cwd);
     const worktrees = await listWorktrees(cwd).catch(() => []);
-    res.json({ available: true, branches, current, worktrees });
+    const remotes = (await listRemoteBranches(cwd).catch(() => [])).map((r) => r.name);
+    res.json({ available: true, branches, remotes, current, worktrees });
   } catch {
-    res.json({ available: false, branches: [], current: null, worktrees: [] });
+    res.json({ available: false, branches: [], remotes: [], current: null, worktrees: [] });
   }
 });
 
@@ -1987,6 +1991,7 @@ router.post("/:id/git/:action", sameOriginGuard, async (req, res) => {
       "EBADMESSAGE",
       "EBADBRANCH",
       "ENOBRANCH",
+      "EAMBIGUOUS",
       "EBRANCHEXISTS",
       "EBRANCHBUSY",
       "EDIRTY",