feat: render flash messages as toasts

Co-Authored-By: Claude Opus 5 <[email protected]>

assets/app.js

@@ -1,4 +1,7 @@
 import './stimulus_bootstrap.js';
+
+// Ahead of app.css: the .toastify--* rules in there override this stylesheet.
+import 'toastify-js/src/toastify.css';
 import './styles/app.css';
 
 import htmx from 'htmx.org';