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/app/App.tsx

@@ -11,6 +11,7 @@ import { ProjectsView } from "@/features/projects";
 import { FirstRunWizard } from "@/features/first-run";
 import { AnnouncementsProvider } from "@/features/announcements";
 import { AppExitConfirmDialog } from "@/features/appExit";
+import { PluginRuntimeProvider } from "@/features/plugins";
 import { Panel, Spinner, Toolbar } from "@/shared";
 import { useGateways, shouldUseMock } from "./di";
 
@@ -65,6 +66,7 @@ export function App() {
 
   return (
     <AnnouncementsProvider>
+    <PluginRuntimeProvider>
     <div className="flex h-full flex-col bg-canvas text-content">
       {/* ── Header ── */}
       <header className="flex shrink-0 items-center justify-between border-b border-border px-6 py-3">
@@ -116,6 +118,7 @@ export function App() {
       </div>
     </div>
     <AppExitConfirmDialog />
+    </PluginRuntimeProvider>
     </AnnouncementsProvider>
   );
 }