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/adapters/index.ts

@@ -34,6 +34,7 @@ import { TauriTicketGateway } from "./ticket";
 import { TauriWindowGateway } from "./window";
 import { TauriFocusedProjectGateway } from "./focusedProject";
 import { LocalStorageUiPreferencesGateway } from "./uiPreferences";
+import { TauriPluginGateway } from "./plugin";
 
 function notImplemented(what: string): never {
   const err: GatewayError = {
@@ -75,6 +76,7 @@ export function createTauriGateways(): Gateways {
     window: new TauriWindowGateway(),
     focusedProject: new TauriFocusedProjectGateway(),
     uiPreferences: new LocalStorageUiPreferencesGateway(),
+    plugin: new TauriPluginGateway(),
   };
 }
 
@@ -101,4 +103,5 @@ export {
   TauriWindowGateway,
   TauriFocusedProjectGateway,
   LocalStorageUiPreferencesGateway,
+  TauriPluginGateway,
 };