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/LaneGitBar.tsx

@@ -1,7 +1,9 @@
 /**
  * @file The lane's git action row: ahead/behind against the upstream, and the
  * writes `POST /api/lanes/:id/git/:action` offers that are not per-file —
- * fetch, pull, push, switch branch, create/delete a branch, publish a repo
+ * fetch, pull, push, switch branch (a local one, or one only a remote has —
+ * listed under "On the remote" and checked out as a new tracking branch; the
+ * list is re-read after every fetch and pull), create/delete a branch, publish a repo
  * that has no remote yet (a URL, then `origin` + `push -u`), and start a merge
  * (the picked branch, or the upstream once diverged; resolving it is the
  * Changes panel's job). Committing is
@@ -114,6 +116,8 @@ export function LaneGitBar({
   const [error, setError] = useState<string | null>(null);
   const [switching, setSwitching] = useState(false);
   const [branches, setBranches] = useState<string[] | null>(null);
+  // Fetched remote branches with no local one yet (`origin/feature`).
+  const [remoteBranches, setRemoteBranches] = useState<string[]>([]);
   const [worktrees, setWorktrees] = useState<LaneWorktree[]>([]);
   const [picked, setPicked] = useState("");
   const [newBranch, setNewBranch] = useState("");
@@ -138,6 +142,7 @@ export function LaneGitBar({
     setError(null);
     setSwitching(false);
     setBranches(null);
+    setRemoteBranches([]);
     setWorktrees([]);
     setPicked("");
     setNewBranch("");
@@ -152,6 +157,9 @@ export function LaneGitBar({
 
   if (!git.available) return null;
   const dirty = git.dirty + git.untracked;
+  // A remote branch: switching creates its local tracking branch; it cannot
+  // be deleted here or given a sibling worktree before that.
+  const pickedRemote = remoteBranches.includes(picked);
   const merging = !!git.merging;
 
   const run = async (action: Action, body?: RunBody) => {
@@ -184,6 +192,9 @@ export function LaneGitBar({
         setPicked("");
         setBranches(null);
         await refreshBranches();
+      } else if ((action === "fetch" || action === "pull") && branches !== null) {
+        // A fetch may have brought branches the list does not show yet.
+        await refreshBranches();
       }
       onChanged();
     } catch (err) {
@@ -217,6 +228,7 @@ export function LaneGitBar({
       const r = await api.lanes.gitBranches(gitTarget(laneId, session, source));
       if (laneNow.current !== laneId) return;
       setBranches(r.branches);
+      setRemoteBranches(r.remotes || []);
       setWorktrees(r.worktrees || []);
     } catch {
       if (laneNow.current === laneId) setBranches([]);
@@ -446,6 +458,15 @@ export function LaneGitBar({
                 </option>
               );
             })}
+            {remoteBranches.length > 0 && (
+              <optgroup label={t("gitBar.remoteBranches")}>
+                {remoteBranches.map((b) => (
+                  <option key={`remote:${b}`} value={b}>
+                    {b}
+                  </option>
+                ))}
+              </optgroup>
+            )}
           </select>
           <Btn
             label={t("gitBar.switchApply")}
@@ -476,7 +497,7 @@ export function LaneGitBar({
                     })
                   : t("gitBar.newLaneHint")
               }
-              disabled={!picked || heldBy.has(picked) || picked === git.branch}
+              disabled={!picked || heldBy.has(picked) || picked === git.branch || pickedRemote}
               // The sibling lane is created through the normal Add-lane flow,
               // prefilled — one provisioning path, not a second one here.
               onClick={() => onNewLane(picked)}
@@ -486,7 +507,7 @@ export function LaneGitBar({
             label={t("gitBar.branchDelete")}
             icon={<Trash2 className="h-3 w-3" />}
             busy={busy === "branch-delete"}
-            disabled={!picked || picked === git.branch || !!heldBy.get(picked)}
+            disabled={!picked || picked === git.branch || !!heldBy.get(picked) || pickedRemote}
             title={t("gitBar.branchDeleteHint")}
             onClick={() =>
               void confirmRun(

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();