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/components/lanes/__tests__/LaneGitBar.test.tsx

@@ -4,7 +4,8 @@
  * buttons a repo with no remote may press (and that it may publish to a typed URL),
  * what an unborn repo and a branch with no upstream show, that it offers no commit (the
  * Changes panel's staged-only box owns that), that a dirty tree blocks the branch switch client-side (the server
- * refuses it too), and that a failed action shows git's own words rather than
+ * refuses it too), that branches only the remote has are offered (and the list
+ * is re-read after a fetch), and that a failed action shows git's own words rather than
  * a generic sentence.
  * @author Nguyễn Ngọc Trí Vĩ <vinnt@smartgift.vn>
  */
@@ -236,6 +237,43 @@ describe("LaneGitBar", () => {
     await waitFor(() => expect(api.lanes.gitBranches).toHaveBeenCalledTimes(2));
   });
 
+  it("lists branches only the remote has, switches to one, and re-reads the list after a fetch", async () => {
+    vi.mocked(api.lanes.gitBranches).mockResolvedValue({
+      available: true,
+      branches: ["development", "feat/thing"],
+      remotes: ["origin/feat/new"],
+      current: "feat/thing",
+      worktrees: [],
+    });
+    const user = userEvent.setup();
+    render(
+      <LaneGitBar
+        laneId={3}
+        git={facts({ dirty: 0, untracked: 0 })}
+        onChanged={vi.fn()}
+        onNewLane={vi.fn()}
+      />
+    );
+    await user.click(screen.getByRole("button", { name: /Branch/ }));
+    const select = await screen.findByLabelText("Switch branch…");
+    expect(
+      screen.getByRole("group", { name: "On the remote — switching creates the local branch" })
+    ).toBeInTheDocument();
+    await user.selectOptions(select, "origin/feat/new");
+    // Not a local branch yet: nothing to delete, no worktree to start on it.
+    expect(screen.getByRole("button", { name: "Delete" })).toBeDisabled();
+    expect(screen.getByRole("button", { name: /New workspace here/ })).toBeDisabled();
+    await user.click(screen.getByRole("button", { name: "Switch branch" }));
+    await waitFor(() =>
+      expect(api.lanes.gitAction).toHaveBeenCalledWith(3, "switch", { branch: "origin/feat/new" })
+    );
+
+    vi.mocked(api.lanes.gitBranches).mockClear();
+    await user.click(screen.getByRole("button", { name: /Fetch/ }));
+    await waitFor(() => expect(api.lanes.gitAction).toHaveBeenCalledWith(3, "fetch", {}));
+    await waitFor(() => expect(api.lanes.gitBranches).toHaveBeenCalledTimes(1));
+  });
+
   it("creates a branch without switching, then offers a sibling lane on it", async () => {
     const user = userEvent.setup();
     const onNewLane = vi.fn();