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/components/workout-form.tsx

@@ -16,6 +16,7 @@ import { TagInput } from "@/components/tag-input"
 import { ExerciseList, Exercise } from "@/components/exercise-list"
 import { MediaUpload } from "@/components/media-upload"
 import { moveFilesFromTempToWorkout } from "@/lib/storage"
+import { toast } from "sonner"
 import type { Database } from "@/lib/supabase"
 
 type Workout = Database["public"]["Tables"]["workouts"]["Row"]
@@ -215,7 +216,7 @@ export default function WorkoutForm({ initialData, mode = "create" }: WorkoutFor
       router.push(`/workouts/${result.data.id}`)
     } catch (err) {
       console.error("Failed to save workout:", err)
-      alert("Failed to save workout. Please try again.")
+      toast.error("Failed to save workout. Please try again.")
     } finally {
       setIsLoading(false)
     }