fix(web): route la famille layout dans /api/invoke + fallback UX restauration — #212

Le web-server ne routait pas load_layout (UNKNOWN_COMMAND a l'ouverture
d'un projet web PC). Alignement de l'adapter web sur les contrats Tauri:
load_layout, mutate_layout, list_layouts, create_layout, rename_layout,
delete_layout, set_active_layout. useLayout ne propage plus l'erreur
brute: message utilisateur + fallback singleLeafTree.

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

@@ -35,6 +35,31 @@ function renderGrid(layout: MockLayoutGateway, projectId = "p1") {
 }
 
 describe("LayoutGrid (with MockLayoutGateway)", () => {
+  it("falls back to an empty workspace with a user-facing message when layout restore fails", async () => {
+    const layout = new MockLayoutGateway();
+    vi.spyOn(layout, "loadLayout").mockRejectedValueOnce(
+      new Error("unknown command: load_layout"),
+    );
+    const warn = vi.spyOn(console, "warn").mockImplementation(() => {});
+
+    renderGrid(layout);
+
+    const alert = await screen.findByRole("alert");
+    expect(alert.textContent).toContain(
+      "Impossible de restaurer l’espace de travail de ce projet.",
+    );
+    expect(alert.textContent).not.toContain("unknown command");
+    await waitFor(() => {
+      expect(screen.getAllByTestId("layout-leaf")).toHaveLength(1);
+    });
+    expect(warn).toHaveBeenCalledWith(
+      "Layout restore failed",
+      expect.objectContaining({ message: "unknown command: load_layout" }),
+    );
+
+    warn.mockRestore();
+  });
+
   it("renders a single-leaf layout without throwing", async () => {
     const layout = new MockLayoutGateway();
     expect(() => renderGrid(layout)).not.toThrow();

frontend/src/features/layout/useLayout.ts

@@ -20,9 +20,13 @@ import { useGateways } from "@/app/di";
 import {
   leaves,
   replaceCustomPluginLayoutWithTerminal,
+  singleLeafTree,
   splitOp,
 } from "./layout";
 
+const LAYOUT_RESTORE_ERROR =
+  "Impossible de restaurer l’espace de travail de ce projet. Un espace vide a été ouvert ; rechargez le projet pour réessayer.";
+
 /** What the layout grid UI needs from this hook. */
 export interface LayoutViewModel {
   /** The current layout tree, or `null` until loaded. */
@@ -109,13 +113,18 @@ export function useLayout(
     }
     let cancelled = false;
     setBusy(true);
+    setError(null);
     gateway
       .loadLayout(projectId, layoutId)
       .then((tree) => {
         if (!cancelled) applyTree(tree);
       })
       .catch((e: unknown) => {
-        if (!cancelled) setError(describe(e));
+        if (!cancelled) {
+          console.warn("Layout restore failed", e);
+          applyTree(singleLeafTree());
+          setError(LAYOUT_RESTORE_ERROR);
+        }
       })
       .finally(() => {
         if (!cancelled) setBusy(false);