feat: render flash messages as toasts

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

tailwind/components/toast.css

@@ -0,0 +1,21 @@
+/* Overrides for toastify-js. Its own stylesheet is vendored through the importmap and imported
+   in assets/app.js ahead of this file, so these rules win without extra specificity. */
+.toastify {
+    @apply rounded shadow-lg text-sm text-white;
+}
+
+.toastify--success {
+    background: theme('colors.emerald.600');
+}
+
+.toastify--warning {
+    background: theme('colors.amber.600');
+}
+
+.toastify--error {
+    background: theme('colors.red.600');
+}
+
+.toastify--info {
+    background: theme('colors.primary');
+}