feat(lanes): full-width Changes, Stage all, one-read expand-all

- The Changes panel's width control gains a full-width step (narrow →
  50/50 → 70/30 → full → closed); the console beside it is hidden with
  CSS, never unmounted.
- "Expand all folders" is one GET /api/lanes/:id/git/tree-all (one
  `git ls-files`, every level shaped like /git/tree) instead of one
  request and one git process per folder.
- Overview: a Stage all button and the staged-only commit box sit above
  the file list and stay pinned while it scrolls; the Files finder is
  pinned too.

client/src/components/lanes/__tests__/LaneChangesPanel.test.tsx

@@ -28,6 +28,7 @@ vi.mock("../../../lib/api", () => ({
       gitFile: vi.fn(),
       gitLs: vi.fn(),
       gitTree: vi.fn(),
+      gitTreeAll: vi.fn(),
     },
   },
 }));
@@ -283,6 +284,22 @@ describe("LaneChangesPanel", () => {
     await waitFor(() => expect(api.lanes.gitLog).toHaveBeenCalledTimes(1));
   });
 
+  it("stages every file the index lacks in one call, from above the list", async () => {
+    const user = userEvent.setup();
+    render(<LaneChangesPanel laneId={7} />);
+    const list = await screen.findByTestId("lane-changes-overview");
+    const actions = screen.getByTestId("lane-changes-actions");
+    // Commit box and Stage all come BEFORE the file list.
+    expect(actions.compareDocumentPosition(list) & Node.DOCUMENT_POSITION_FOLLOWING).toBeTruthy();
+    expect(screen.getByTestId("lane-changes-commit")).toBeInTheDocument();
+
+    await user.click(screen.getByRole("button", { name: "Stage all (2)" }));
+    // The already-staged docs/API.md is not sent again.
+    expect(api.lanes.gitAction).toHaveBeenCalledWith(7, "stage", {
+      paths: ["server/lib/git-ops.js", "notes.txt"],
+    });
+  });
+
   it("needs two clicks to discard, and only the second one reaches the server", async () => {
     const user = userEvent.setup();
     render(<LaneChangesPanel laneId={7} />);
@@ -417,8 +434,16 @@ describe("LaneChangesPanel", () => {
     expect(screen.getByText("src")).toBeInTheDocument();
   });
 
-  it("expands every folder at once, fetching whatever level wasn't cached yet", async () => {
+  it("expands every folder at once from one whole-tree read", async () => {
     const user = userEvent.setup();
+    vi.mocked(api.lanes.gitTreeAll).mockResolvedValue({
+      available: true,
+      truncated: false,
+      levels: {
+        "": [{ name: "src", path: "src", dir: true }],
+        src: [{ name: "main.ts", path: "src/main.ts", dir: false }],
+      },
+    });
     vi.mocked(api.lanes.gitTree).mockImplementation(async (_id, path) => {
       if (path === "src")
         return {
@@ -788,37 +813,29 @@ describe("LaneChangesPanel", () => {
     expect(screen.getByText("server/lib/git-ops.js")).toBe(before);
   });
 
-  it("expands all folders with at most four tree reads in flight", async () => {
+  it("expands all folders with ONE request, not one per folder", async () => {
     const user = userEvent.setup();
-    let inFlight = 0;
-    let peak = 0;
     vi.mocked(api.lanes.gitTree).mockClear();
-    vi.mocked(api.lanes.gitTree).mockImplementation(async (_id, path = "") => {
-      inFlight += 1;
-      peak = Math.max(peak, inFlight);
-      await new Promise((r) => setTimeout(r, 5));
-      inFlight -= 1;
-      const depth = path ? path.split("/").length : 0;
-      const entries =
-        depth >= 2
-          ? [{ name: "leaf.ts", path: `${path}/leaf.ts`, dir: false }]
-          : Array.from({ length: 5 }, (_, i) => ({
-              name: `d${i}`,
-              path: path ? `${path}/d${i}` : `d${i}`,
-              dir: true,
-            }));
-      return { available: true, path, entries, truncated: false };
-    });
+    vi.mocked(api.lanes.gitTreeAll)
+      .mockClear()
+      .mockResolvedValue({
+        available: true,
+        truncated: false,
+        levels: {
+          "": [{ name: "src", path: "src", dir: true }],
+          src: [{ name: "main.ts", path: "src/main.ts", dir: false }],
+        },
+      });
     render(<LaneChangesPanel laneId={7} />);
     await screen.findByTestId("lane-changes-overview");
     await user.click(screen.getByRole("button", { name: "Files" }));
-    await screen.findByText("d0");
+    await screen.findByText("src");
     await user.click(screen.getByRole("button", { name: "Expand all folders" }));
 
-    await waitFor(() => expect(screen.getAllByText("leaf.ts")).toHaveLength(25));
-    // Root + 5 + 25 folders, each fetched exactly once.
-    expect(api.lanes.gitTree).toHaveBeenCalledTimes(31);
-    expect(peak).toBeLessThanOrEqual(4);
+    expect(await screen.findByText("main.ts")).toBeInTheDocument();
+    expect(api.lanes.gitTreeAll).toHaveBeenCalledTimes(1);
+    // Only the root read the Files tab made on entry — no per-folder walk.
+    expect(api.lanes.gitTree).toHaveBeenCalledTimes(1);
   });
 
   it("does not repaint the previous lane after its git action resolves post-switch", async () => {
@@ -861,21 +878,25 @@ describe("LaneChangesPanel", () => {
     expect(screen.queryByText("server/lib/git-ops.js")).toBeNull();
   });
 
-  it("does not paint the previous lane's tree when an expand-all walk outlives a lane switch", async () => {
+  it("does not paint the previous lane's tree when an expand-all read outlives a lane switch", async () => {
     const user = userEvent.setup();
     let resolveSrc: () => void = () => {};
-    vi.mocked(api.lanes.gitTree).mockImplementation((id, path) => {
-      if (id === 7 && path === "src")
-        return new Promise(
+    vi.mocked(api.lanes.gitTreeAll).mockImplementation(
+      () =>
+        new Promise(
           (r) =>
             (resolveSrc = () =>
               r({
                 available: true,
-                path: "src",
-                entries: [{ name: "main.ts", path: "src/main.ts", dir: false }],
                 truncated: false,
+                levels: {
+                  "": [{ name: "src", path: "src", dir: true }],
+                  src: [{ name: "main.ts", path: "src/main.ts", dir: false }],
+                },
               }))
-        );
+        )
+    );
+    vi.mocked(api.lanes.gitTree).mockImplementation((id) => {
       return Promise.resolve({
         available: true,
         path: "",