Add persistent action status banner and fix stacked toast overlap

Stack actions (start/stop/pull/update/…) now surface a dismissible
status banner on the stack detail page instead of relying on the
transient top-right toast alone. Also enable toast expand mode so
multiple notifications no longer collapse behind each other.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_016pMmFFkdfxkoYjcEcpZTa5

frontend/src/main.tsx

@@ -13,7 +13,7 @@ ReactDOM.createRoot(document.getElementById("root")!).render(
   <React.StrictMode>
     <QueryClientProvider client={queryClient}>
       <App />
-      <Toaster position="top-right" richColors theme="system" />
+      <Toaster position="top-right" richColors theme="system" expand visibleToasts={5} />
     </QueryClientProvider>
   </React.StrictMode>
 );