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/http/index.ts

@@ -45,6 +45,7 @@ import {
 import {
   WebDesktopServerGateway,
   WebFocusedProjectGateway,
+  WebPluginGateway,
   WebRemoteGateway,
   WebWindowGateway,
 } from "./unsupported";
@@ -139,6 +140,7 @@ export function createHttpWsGateways(config: HttpWsGatewaysConfig = {}): Gateway
     focusedProject: new WebFocusedProjectGateway(),
     // Frontend-owned UI prefs are transport-neutral (localStorage) — reuse as-is.
     uiPreferences: new LocalStorageUiPreferencesGateway(),
+    plugin: new WebPluginGateway(),
   };
 }