fix: show toasts on every screen, including the Unified Inbox and contacts

Only the tab screens and the viewer had a toast host, so a viewer
action that failed after going back to the Unified Inbox or a contact
(both stack screens of their own) raised a toast nobody saw.

Every stack screen now gets a host through the navigator's
`screenLayout`, and only the focused screen's host renders, so the
screen kept live under the top one never shows the same toast a second
time. The tab screens' and the viewer's own hosts are gone.

src/components/ToastHost.tsx

@@ -9,9 +9,10 @@ import { useToastStore, type Toast } from '../stores/toast-store';
 import { useLocaleStore } from '../stores/locale-store';
 
 /**
- * Renders the toast queue above the tab bar. One host is mounted in App.tsx;
- * the email undo snackbar keeps its own component because it is bound to
- * the email store's pending-undo entry.
+ * Renders the toast queue above the tab bar. App.tsx gives every stack screen
+ * one and only the focused screen's renders; the email undo snackbar keeps
+ * its own component because it is bound to the email store's pending-undo
+ * entry.
  */
 export function ToastHost(): React.ReactElement | null {
   const toasts = useToastStore((s) => s.toasts);