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/usePluginMenus.ts

@@ -0,0 +1,63 @@
+/**
+ * `usePluginMenus` — turns the loaded plugin runtime registry into ready-to-
+ * splice `MenuBarMenu`/`MenuBarItem` entries for `ProjectsView`'s menu bar
+ * (ticket #43, F3, carnet §7.1/§7.2).
+ *
+ * `whenCtx` is supplied by the caller (`ProjectsView` knows `projectOpen`
+ * directly; git/agent/terminal/cell-focus signals are best-effort — see the
+ * F3 gap noted in the delivery report: there is no existing global
+ * agent-selected/terminal-focused/layout-cell-focused signal at the menu-bar
+ * level in this codebase today, so those three default to `false` until a
+ * future lot threads real focus state up to this level).
+ */
+
+import { useMemo } from "react";
+
+import type { MenuTargetId } from "@/domain";
+import type { MenuBarItem, MenuBarMenu } from "@/shared";
+import type { PluginRuntimeRegistry, WhenContext } from "@/plugins/runtime";
+import { resolveMenuItems, resolveTopLevelMenus } from "./menus";
+
+export interface UsePluginMenusResult {
+  /** Top-level plugin menus, in order, ready to splice between Panneaux/Paramètres. */
+  topLevelMenus: MenuBarMenu[];
+  /** Resolved+ordered `MenuBarItem`s to append to a native menu's items. */
+  itemsFor: (targetMenuId: MenuTargetId) => MenuBarItem[];
+  runCommand: (pluginId: string, commandId: string) => Promise<void>;
+}
+
+export function usePluginMenus(
+  registry: PluginRuntimeRegistry,
+  whenCtx: WhenContext,
+): UsePluginMenusResult {
+  const runCommand = useMemo(
+    () => (pluginId: string, commandId: string) => registry.runCommand(pluginId, commandId),
+    [registry],
+  );
+
+  const itemsFor = useMemo(() => {
+    return (targetMenuId: MenuTargetId): MenuBarItem[] =>
+      resolveMenuItems(registry, targetMenuId, whenCtx).map((item) => ({
+        id: item.id,
+        // Provenance is shown discreetly (carnet §0 UX decision) rather than
+        // with a visual section divider, which the shared `MenuBar` primitive
+        // doesn't support — a trailing "· Plugin Name" suffix is the least
+        // intrusive option that still surfaces where the item came from.
+        label: `${item.label} · ${item.pluginDisplayName}`,
+        disabled: !item.enabled,
+        onSelect: () => {
+          void runCommand(item.pluginId, item.command);
+        },
+      }));
+  }, [registry, whenCtx, runCommand]);
+
+  const topLevelMenus = useMemo<MenuBarMenu[]>(() => {
+    return resolveTopLevelMenus(registry).map((menu) => ({
+      id: menu.id,
+      label: menu.label,
+      items: itemsFor(menu.id),
+    }));
+  }, [registry, itemsFor]);
+
+  return { topLevelMenus, itemsFor, runCommand };
+}