feat: replace inline save status with toast notifications in settings

Add a standalone toast notification system (no Provider needed) and use it
for settings save feedback. Skip save when fields are empty. Remove save
button on Anilist local user select in favor of auto-save on change.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

apps/backoffice/app/globals.css

@@ -226,6 +226,15 @@ body::after {
   animation: fade-in 0.3s ease-in;
 }
 
+@keyframes slide-in-right {
+  from { opacity: 0; transform: translateX(100%); }
+  to { opacity: 1; transform: translateX(0); }
+}
+
+.animate-slide-in-right {
+  animation: slide-in-right 0.3s ease-out;
+}
+
 /* Line clamp utilities */
 .line-clamp-1 {
   display: -webkit-box;