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/unsupported.ts

@@ -12,6 +12,11 @@
 import type {
   EmbeddedServerStatus,
   GatewayError,
+  PluginAdmin,
+  PluginInstallResult,
+  PluginReview,
+  PluginRuntimeContributionCatalog,
+  PluginUninstallResult,
   ServerExposurePreview,
   ServerExposureSettings,
   Unsubscribe,
@@ -20,7 +25,9 @@ import type {
   DesktopServerGateway,
   FocusedProject,
   FocusedProjectGateway,
+  PluginGateway,
   RemoteGateway,
+  ReviewPluginPackageInput,
   ViewWindowClosed,
   ViewWindowSnapshot,
   WindowGateway,
@@ -119,3 +126,37 @@ export class WebDesktopServerGateway implements DesktopServerGateway {
     return Promise.resolve(() => {});
   }
 }
+
+/**
+ * Web stub: the plugin system (#43) installs/loads full-trust local ESM
+ * bundles from the desktop filesystem — no server-side equivalent in V1. The
+ * web client must not manage or load plugins on behalf of the desktop app.
+ */
+export class WebPluginGateway implements PluginGateway {
+  async listPlugins(): Promise<PluginAdmin[]> {
+    return unsupportedOnWeb("Plugin management");
+  }
+  async reviewPackage(_input: ReviewPluginPackageInput): Promise<PluginReview> {
+    return unsupportedOnWeb("Plugin management");
+  }
+  async installFromArchive(_path: string): Promise<PluginInstallResult> {
+    return unsupportedOnWeb("Plugin management");
+  }
+  async installFromDirectory(_path: string): Promise<PluginInstallResult> {
+    return unsupportedOnWeb("Plugin management");
+  }
+  async setEnabled(_pluginId: string, _enabled: boolean): Promise<PluginAdmin> {
+    return unsupportedOnWeb("Plugin management");
+  }
+  async uninstall(_pluginId: string): Promise<PluginUninstallResult> {
+    return unsupportedOnWeb("Plugin management");
+  }
+  async listRuntimeContributions(): Promise<PluginRuntimeContributionCatalog> {
+    // No plugin bundles are ever loaded on the web client — an empty catalog
+    // lets the bootstrap loader run unconditionally without a transport check.
+    return { plugins: [] };
+  }
+  async openPluginsFolder(_pluginId?: string): Promise<void> {
+    return unsupportedOnWeb("Plugin management");
+  }
+}