feat: add admin page

lib/viewmodels/admin_pin_view_model.dart

@@ -0,0 +1,112 @@
+import 'package:flutter/foundation.dart';
+
+import '../services/admin_pin_service.dart';
+
+class AdminPinViewModel extends ChangeNotifier {
+  final AdminPinService adminPinService;
+
+  AdminPinViewModel({required this.adminPinService});
+
+  bool _isPinSet = false;
+  bool _isUnlocked = false;
+  bool _isLoading = false;
+  bool _hasLoaded = false;
+  String? _errorMessage;
+
+  bool get isPinSet => _isPinSet;
+
+  bool get isUnlocked => _isUnlocked;
+
+  bool get isLoading => _isLoading;
+
+  bool get hasLoaded => _hasLoaded;
+
+  String? get errorMessage => _errorMessage;
+
+  Future<void> ensureLoaded() async {
+    if (_hasLoaded || _isLoading) return;
+
+    await load();
+  }
+
+  Future<void> load() async {
+    if (_isLoading) return;
+
+    _isLoading = true;
+    _errorMessage = null;
+    notifyListeners();
+
+    try {
+      _isPinSet = await adminPinService.hasPin();
+    } catch (e) {
+      debugPrint('AdminPinViewModel: load error: $e');
+      _errorMessage = 'Could not check PIN status.';
+    } finally {
+      _hasLoaded = true;
+      _isLoading = false;
+      notifyListeners();
+    }
+  }
+
+  Future<bool> setPin(String pin) async {
+    _errorMessage = null;
+
+    try {
+      await adminPinService.setPin(pin);
+      _isPinSet = true;
+      _isUnlocked = true;
+      notifyListeners();
+      return true;
+    } catch (e) {
+      debugPrint('AdminPinViewModel: setPin error: $e');
+      _errorMessage = 'Could not save PIN.';
+      notifyListeners();
+      return false;
+    }
+  }
+
+  Future<bool> verify(String pin) async {
+    _errorMessage = null;
+
+    try {
+      final matches = await adminPinService.verifyPin(pin);
+
+      if (matches) {
+        _isUnlocked = true;
+        notifyListeners();
+        return true;
+      }
+
+      _errorMessage = 'Incorrect PIN.';
+      notifyListeners();
+      return false;
+    } catch (e) {
+      debugPrint('AdminPinViewModel: verify error: $e');
+      _errorMessage = 'Could not verify PIN.';
+      notifyListeners();
+      return false;
+    }
+  }
+
+  Future<bool> changePin({
+    required String currentPin,
+    required String newPin,
+  }) async {
+    final currentMatches = await adminPinService.verifyPin(currentPin);
+
+    if (!currentMatches) {
+      _errorMessage = 'Current PIN is incorrect.';
+      notifyListeners();
+      return false;
+    }
+
+    return setPin(newPin);
+  }
+
+  void lock() {
+    if (!_isUnlocked) return;
+
+    _isUnlocked = false;
+    notifyListeners();
+  }
+}