feat(ui): fenêtres View système séparées — WebviewWindow Tauri + port WindowGateway (#23)

Détache les vues dans de vraies fenêtres système Tauri (multi-écran,
fullscreen). Backend : commandes de gestion de WebviewWindow, capabilities
et composition root. Frontend : nouveau port WindowGateway et son adaptateur
window, entrée panel-only ViewWindow/ViewPanelBody, détachement câblé dans
ProjectsView. QA vert : app-tauri 63, frontend typecheck + vitest 546/546.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

frontend/src/adapters/index.ts

@@ -28,6 +28,7 @@ import { TauriPermissionGateway } from "./permission";
 import { TauriWorkStateGateway } from "./workState";
 import { TauriConversationGateway } from "./conversation";
 import { TauriTicketGateway } from "./ticket";
+import { TauriWindowGateway } from "./window";
 
 function notImplemented(what: string): never {
   const err: GatewayError = {
@@ -63,6 +64,7 @@ export function createTauriGateways(): Gateways {
     workState: new TauriWorkStateGateway(),
     conversation: new TauriConversationGateway(),
     ticket: new TauriTicketGateway(),
+    window: new TauriWindowGateway(),
   };
 }
 
@@ -83,4 +85,5 @@ export {
   TauriWorkStateGateway,
   TauriConversationGateway,
   TauriTicketGateway,
+  TauriWindowGateway,
 };