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/system.ts

@@ -41,6 +41,15 @@ export class TauriSystemGateway implements SystemGateway {
     return typeof result === "string" ? result : null;
   }
 
+  async pickArchiveFile(): Promise<string | null> {
+    const result = await open({
+      directory: false,
+      multiple: false,
+      filters: [{ name: "Plugin archive", extensions: ["ideaplug", "zip", "vsix"] }],
+    });
+    return typeof result === "string" ? result : null;
+  }
+
   async onAppExitWorkGuard(
     handler: (state: AppExitWorkGuardState) => void,
   ): Promise<Unsubscribe> {