fix(app-tauri,frontend): le panneau MCP se détache en fenêtre OS — #299 (cause: enum Rust ViewPanel sans variante Mcp, open_view_window("mcp") rejetait INVALID et detachPanel ne touchait le placement qu'en cas de succès donc aucun effet visible ni message; fix: variante ViewPanel::Mcp avec parse/as_str/title "MCP" + contrat stable label view-mcp / url index.html?panel=mcp / decodage view_panel_from_window_label, identite persistee des fenetres view-mcp acceptee a la restauration #50, et ProjectsView surface desormais un echec de detachement via alerte role=alert avec bouton Reessayer au lieu de l'echec silencieux — placement conserve tant que la fenetre OS ne s'ouvre pas; QA verte reconfirmee par Git: rustup run stable cargo fmt --all -- --check, git diff --check, cargo test -p app-tauri view_window_tests 9/9 dont mcp_view_window_contract_is_stable, persisted_identity_accepts_mcp_view_label 1/1, npm test 22/22 sur ViewWindow/ProjectsView.detach/viewPlacement, npm run build tsc+vite OK; fichiers tickets/296/*, tickets/299/, scripts/ et sync counter/index hors perimetre; aucun push)

frontend/src/features/projects/ProjectsView.tsx

@@ -246,6 +246,7 @@ export function ProjectsView() {
   const [pendingConversationOpen, setPendingConversationOpen] =
     useState<PendingConversationOpen | null>(null);
   const [taskToasts, setTaskToasts] = useState<BackgroundTaskToast[]>([]);
+  const [detachErrorPanel, setDetachErrorPanel] = useState<PanelId | null>(null);
 
   const active = vm.openTabs.find((t) => t.id === vm.activeTabId) ?? null;
   const activeViewerConversation =
@@ -446,11 +447,17 @@ export function ProjectsView() {
   // shell. On failure the placement is left untouched (no ghost "detached" slot).
   function detachPanel(panel: PanelId) {
     if (!active) return;
+    setDetachErrorPanel(null);
     void windowGateway
       .openViewWindow(panel)
-      .then(() => setPlacement(panel, "detached"))
+      .then(() => {
+        setDetachErrorPanel(null);
+        setPlacement(panel, "detached");
+      })
       .catch(() => {
-        /* window failed to open; keep the current placement */
+        // Keep the current placement (no ghost detached slot), but surface the
+        // failure so the action never appears to do nothing (#299).
+        setDetachErrorPanel(panel);
       });
   }
 
@@ -868,6 +875,20 @@ export function ProjectsView() {
           {vm.error}
         </p>
       )}
+      {detachErrorPanel && (
+        <div
+          role="alert"
+          className="mx-4 mt-2 flex shrink-0 items-center justify-between gap-3 rounded-md border border-danger/40 bg-danger/10 px-3 py-2 text-sm text-danger"
+        >
+          <span>
+            Le panneau {PANEL_TITLE[detachErrorPanel]} ne peut pas être détaché
+            pour le moment.
+          </span>
+          <Button size="sm" onClick={() => detachPanel(detachErrorPanel)}>
+            Réessayer
+          </Button>
+        </div>
+      )}
 
       {/* ── Project tab bar (#26): tabs = open projects, "+" opens Projects ── */}
       <ProjectTabs