Clear demo data, namespace storage by user, add profile setup

- Remove all personal sampleTasks from TaskViewModel (tasks start empty)
- Namespace UserDefaults keys by Apple ID for task and workout data isolation
- Add ProfileSetupView for name capture after first Sign in with Apple
- Add updateDisplayName() to AuthManager
- Route to ProfileSetupView when user has no display name yet

Co-Authored-By: Kutesir <tqwyy79vzn@privaterelay.appleid.com>

KisaniCal/Managers/AuthManager.swift

@@ -59,6 +59,14 @@ final class AuthManager: NSObject, ObservableObject {
         }
     }
 
+    // MARK: - Profile
+
+    func updateDisplayName(_ name: String) {
+        guard var user = currentUser, !name.trimmingCharacters(in: .whitespaces).isEmpty else { return }
+        user.displayName = name.trimmingCharacters(in: .whitespaces)
+        persist(user)
+    }
+
     // MARK: - Sign Out
 
     func signOut() {