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/streamGateways.ts

@@ -116,6 +116,11 @@ export class HttpSystemGateway implements SystemGateway {
     return unsupportedOnWeb("Native folder picker");
   }
 
+  pickArchiveFile(): Promise<string | null> {
+    // Desktop-only, same rationale as `pickFolder`.
+    return unsupportedOnWeb("Native file picker");
+  }
+
   onAppExitWorkGuard(
     _handler: (state: AppExitWorkGuardState) => void,
   ): Promise<Unsubscribe> {