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/ports/index.ts

@@ -881,6 +881,44 @@ export interface TicketGateway {
   ): Promise<void>;
 }
 
+/**
+ * Payload of the OS window-lifecycle event that fires when a detached view
+ * window closes (ticket #23). Lets the main window re-toggle the view's
+ * placement out of `"detached"`.
+ */
+export interface ViewWindowClosed {
+  /** The detached panel id (a `PanelId`, kept as `string` at the port seam). */
+  panel: string;
+  /** The project the detached window was showing. */
+  projectId: string;
+}
+
+/**
+ * Detaching a View into its own OS window (ticket #23).
+ *
+ * The backend owns the Tauri `WebviewWindow` lifecycle (create/focus/close,
+ * anti-duplicate registry). This port is the UI's only door to it — components
+ * never call `invoke()` directly. `panel` is a `PanelId`-shaped string; it is
+ * typed as `string` here so ports need not depend on the `features/` layer.
+ */
+export interface WindowGateway {
+  /**
+   * Opens — or focuses, if already open — a separate OS window rendering only
+   * `panel` for `projectId` (the backend's `open_view_window` command).
+   */
+  openViewWindow(panel: string, projectId: string): Promise<void>;
+  /** Closes the detached window for `panel`/`projectId` if one is open. */
+  closeViewWindow(panel: string, projectId: string): Promise<void>;
+  /**
+   * Subscribes to detached-window close events (OS close or programmatic). The
+   * handler receives the `{ panel, projectId }` that closed. Returns an
+   * unsubscribe.
+   */
+  onViewWindowClosed(
+    handler: (event: ViewWindowClosed) => void,
+  ): Promise<Unsubscribe>;
+}
+
 /**
  * The full set of gateways the app depends on, injected via the DI provider.
  * The composition (real vs mock) is chosen in `app/`.
@@ -903,4 +941,5 @@ export interface Gateways {
   workState: WorkStateGateway;
   conversation: ConversationGateway;
   ticket: TicketGateway;
+  window: WindowGateway;
 }