feat(frontend): système de plugins — runtime, menus, layouts custom (#43)

Lots F1-F4 : runtime de chargement/registre plugin, extension des menus
existants, panneau de gestion des plugins, types de layout custom
(sélecteur, fallback, cellule dédiée) branchés sur le port plugin.
Suite npm typecheck/test verte (947/947).

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

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

@@ -0,0 +1,69 @@
+/**
+ * F4 — `PluginLayoutSelectorSection` (carnet §10 F4 acceptance criteria:
+ * "layout disabled non proposé comme nouveau choix").
+ */
+import { describe, expect, it } from "vitest";
+import { render, screen, fireEvent } from "@testing-library/react";
+
+import {
+  PluginCommandRegistry,
+  PluginLayoutRegistry,
+  PluginMenuRegistry,
+  PluginRuntimeRegistry,
+  type LoadedPlugin,
+} from "@/plugins/runtime";
+import { PluginLayoutSelectorSection, listPluginLayoutChoices } from "./PluginLayoutSelectorSection";
+
+function stubPlugin(pluginId: string, displayName: string, layoutType: string): LoadedPlugin {
+  const contributes = {
+    menus: [],
+    menuItems: [],
+    layouts: [{ type: layoutType, label: `${displayName} layout`, component: "X" }],
+    mcpServers: [],
+  };
+  return {
+    pluginId,
+    displayName,
+    contributes,
+    commands: new PluginCommandRegistry(pluginId, new Set()),
+    layouts: new PluginLayoutRegistry(pluginId, new Set([layoutType])),
+    menu: new PluginMenuRegistry(pluginId),
+    dispose: async () => {},
+  };
+}
+
+describe("listPluginLayoutChoices / PluginLayoutSelectorSection", () => {
+  it("only lists loaded (enabled) plugins' layouts — a disabled plugin is never in the registry", () => {
+    const registry = new PluginRuntimeRegistry();
+    registry.add(stubPlugin("dev.acme.one", "One", "dev.acme.one.layout"));
+    // "dev.acme.two" is disabled ⇒ never loaded ⇒ never added to the registry
+    // (carnet §1.3) — nothing to filter here beyond what's already loaded.
+
+    const choices = listPluginLayoutChoices(registry);
+    expect(choices).toHaveLength(1);
+    expect(choices[0].pluginId).toBe("dev.acme.one");
+  });
+
+  it("renders nothing when there are no plugin layouts", () => {
+    const { container } = render(
+      <PluginLayoutSelectorSection registry={new PluginRuntimeRegistry()} onSelect={() => {}} />,
+    );
+    expect(container.innerHTML).toBe("");
+  });
+
+  it("calls onSelect with the chosen plugin layout", () => {
+    const registry = new PluginRuntimeRegistry();
+    registry.add(stubPlugin("dev.acme.one", "One", "dev.acme.one.layout"));
+    let selected: string | null = null;
+    render(
+      <PluginLayoutSelectorSection
+        registry={registry}
+        onSelect={(choice) => {
+          selected = choice.layout.type;
+        }}
+      />,
+    );
+    fireEvent.click(screen.getByRole("menuitem", { name: /One layout/ }));
+    expect(selected).toBe("dev.acme.one.layout");
+  });
+});