fix(plugins): isole les contributions plugin en erreur et durcit menus.ts

Le chargement de l'archive hello-plugin (build/hello-plugin-0.1.0.zip)
vidait la fenêtre principale : une contribution plugin fautive
remontait jusqu'au rendu global au lieu de rester locale à la cellule.
Ajoute un boundary local dans PluginLayoutCellView/PluginLayoutSelectorSection
et durcit menus.ts/loader.ts/registry.ts contre les entrées de menu ou
contributions malformées.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

frontend/src/features/plugins/PluginLayoutSelectorSection.test.tsx

@@ -12,6 +12,7 @@ import {
   PluginRuntimeRegistry,
   type LoadedPlugin,
 } from "@/plugins/runtime";
+import type { PluginContributionDto } from "@/domain";
 import { PluginLayoutSelectorSection, listPluginLayoutChoices } from "./PluginLayoutSelectorSection";
 
 function stubPlugin(pluginId: string, displayName: string, layoutType: string): LoadedPlugin {
@@ -66,4 +67,32 @@ describe("listPluginLayoutChoices / PluginLayoutSelectorSection", () => {
     fireEvent.click(screen.getByRole("menuitem", { name: /One layout/ }));
     expect(selected).toBe("dev.acme.one.layout");
   });
+
+  it("skips malformed layout contributions instead of throwing during sorting", () => {
+    const registry = new PluginRuntimeRegistry();
+    const contributes = {
+      menus: [],
+      menuItems: [],
+      layouts: [
+        { type: "dev.acme.good", label: "Good layout", component: "X" },
+        { type: "dev.acme.no-label", label: undefined, component: "X" },
+        { type: undefined, label: "No type", component: "X" },
+      ] as unknown as PluginContributionDto["layouts"],
+      mcpServers: [],
+    };
+    registry.add({
+      pluginId: "dev.acme.bad",
+      displayName: undefined as unknown as string,
+      contributes,
+      commands: new PluginCommandRegistry("dev.acme.bad", new Set()),
+      layouts: new PluginLayoutRegistry("dev.acme.bad", new Set()),
+      menu: new PluginMenuRegistry("dev.acme.bad"),
+      dispose: async () => {},
+    });
+
+    const choices = listPluginLayoutChoices(registry);
+    expect(choices).toHaveLength(1);
+    expect(choices[0].layout.label).toBe("Good layout");
+    expect(choices[0].pluginDisplayName).toBe("dev.acme.bad");
+  });
 });