feat: implement beautiful toast notifications with Sonner

- Install sonner package for toast notifications
- Add Toaster component to root layout with dark theme styling
- Replace all alert() calls with toast.success() and toast.error()
- Add descriptive messages for success states
- Toast notifications match the app's dark theme design

admin-web/app/trainers/page.tsx

@@ -6,6 +6,7 @@ import { Button } from "@/components/ui/button";
 import { Card, CardContent } from "@/components/ui/card";
 import { Plus, Trash2, Edit, Loader2, Users, AlertCircle } from "lucide-react";
 import Link from "next/link";
+import { toast } from "sonner";
 import type { Database } from "@/lib/supabase";
 
 type Trainer = Database["public"]["Tables"]["trainers"]["Row"];
@@ -56,9 +57,12 @@ export default function TrainersPage() {
       const { error } = await supabase.from("trainers").delete().eq("id", id);
       if (error) throw error;
       setTrainers(trainers.filter((t) => t.id !== id));
+      toast.success("Trainer deleted", {
+        description: "The trainer has been removed successfully."
+      });
     } catch (error) {
       console.error("Failed to delete trainer:", error);
-      alert("Failed to delete trainer");
+      toast.error("Failed to delete trainer");
     } finally {
       setDeletingId(null);
     }