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/pages/__tests__/Workspace.changesPanel.test.tsx

@@ -142,7 +142,7 @@ describe("Workspace — the Changes panel's width", () => {
     }
   }
 
-  it("steps narrow → 50/50 → 70/30 → closed on one control", async () => {
+  it("steps narrow → 50/50 → 70/30 → full → closed on one control", async () => {
     await openPanel();
     const card = () => screen.getByTestId("lane-changes-card");
     expect(card().className).toContain("w-[28rem]");
@@ -153,6 +153,11 @@ describe("Workspace — the Changes panel's width", () => {
     fireEvent.click(screen.getByTestId("lane-changes-resize"));
     expect(card().className).toContain("w-[70%]");
 
+    // Full width HIDES the console beside it — hidden, still mounted.
+    fireEvent.click(screen.getByTestId("lane-changes-resize"));
+    expect(card().className).toContain("w-full");
+    expect(screen.getByTestId("lane-detail").className).toContain("hidden");
+
     // Past the widest step the panel closes — and reopens narrow, not back at
     // 70/30, which is why closing resets the width.
     fireEvent.click(screen.getByTestId("lane-changes-resize"));