fix(layout): barre supérieure CLI custom cliquable — stacking context du corps de leaf

Le corps d'une leaf (layout-leaf-body) crée désormais son propre stacking context (z-index 0). Les vues CLI custom peuvent comporter des enfants à haut z local (toolbar z-20) qui restaient au-dessus des contrôles de cellule (CELL_Z 5) et captaient les clics ; ils restent désormais sous les contrôles. Test de layering dédié ajouté dans LayoutGrid.cellControlsLayering.test.tsx.

frontend/src/features/layout/LayoutGrid.cellControlsLayering.test.tsx

@@ -206,6 +206,29 @@ describe("LayoutGrid — ticket #48 cell control layering", () => {
     expect(controls.style.overflow).toBe("hidden");
   });
 
+  it("keeps cell controls above the custom CLI view stacking context", async () => {
+    const setup = await makeSplitAgentGrid();
+    window.localStorage.setItem(`idea.agent-cell-mode.p1.${setup.bId}`, "custom");
+
+    renderGrid(setup.gateways);
+
+    await screen.findByTestId("custom-agent-chat-view");
+    window.localStorage.removeItem(`idea.agent-cell-mode.p1.${setup.bId}`);
+    const leafBody = screen.getAllByTestId("layout-leaf-body").find((element) =>
+      element.closest(`[data-node-id="${setup.bId}"]`),
+    ) as HTMLElement;
+    const customToolbar = screen.getByRole("toolbar", {
+      name: "custom agent chat actions",
+    });
+    const { controls } = controlsFor(setup.bId);
+
+    expect(leafBody).toBeTruthy();
+    expect(leafBody.style.position).toBe("relative");
+    expect(leafBody.style.zIndex).toBe("0");
+    expect(customToolbar.className).toContain("z-20");
+    expect(controls.style.zIndex).toBe("5");
+  });
+
   it("keeps busy Cancel above persistent cell controls and removes History (#151, #160)", async () => {
     const setup = await makeSplitAgentGrid();
     await setup.layout.mutateLayout("p1", {

frontend/src/features/layout/LayoutGrid.tsx

@@ -338,6 +338,10 @@ function goToCell(nodeId: string): void {
  *   full-cell veils   4   (write-portal + F3 overlays; mutually exclusive)
  *   cell controls     5   (always on top and clickable)
  *   turn actions      6   (busy-state actions; Cancel must stay visible)
+ *
+ * The terminal/chat body must create its own z-index 0 stacking context. Custom
+ * CLI views can legitimately contain local high-z children (for example their
+ * own toolbar), but those children must stay below the cell controls.
  */
 const CELL_Z = {
   banner: 3,
@@ -1300,8 +1304,10 @@ function LeafView({
           navigating. The agent cell passes its {@link WritePortal} so keystroke
           counting + injection suspension are wired; a plain cell passes none. */}
       <div
+        data-testid="layout-leaf-body"
         style={{
           position: "relative",
+          zIndex: 0,
           width: "100%",
           height: "100%",
           minHeight: 0,