Add SettingsViewModel for Managing Settings State

This commit introduces the `SettingsViewModel` class, part of the MVVM architecture, responsible for managing the state and business logic for the settings feature of the app. This ViewModel is associated with the `SettingsView`.

Features Managed by this ViewModel:
- Dark Mode: Toggle the state of Dark Mode and notify listeners.
- Developer Mode: Toggle the state of Developer Mode and notify listeners.
- Base URL: Update the state of Base URL and notify listeners.
- Continuous Mode Steps: Increment and decrement the number of Continuous Mode Steps and notify listeners.

Each change in the state is followed by a notification to the listeners to rebuild the UI components that depend on this state, ensuring a reactive UI. To-do comments have been added where the state needs to be persisted or synchronized with a server.

frontend/lib/viewmodels/settings_viewmodel.dart

@@ -0,0 +1,55 @@
+import 'package:flutter/material.dart';
+
+/// [SettingsViewModel] is responsible for managing the state and logic
+/// for the [SettingsView]. It extends [ChangeNotifier] to provide
+/// reactive state management.
+class SettingsViewModel extends ChangeNotifier {
+  bool _isDarkModeEnabled = false; // State for Dark Mode
+  bool _isDeveloperModeEnabled = false; // State for Developer Mode
+  String _baseURL = ''; // State for Base URL
+  int _continuousModeSteps = 1; // State for Continuous Mode Steps
+
+  // Getters to access the private state variables
+  bool get isDarkModeEnabled => _isDarkModeEnabled;
+  bool get isDeveloperModeEnabled => _isDeveloperModeEnabled;
+  String get baseURL => _baseURL;
+  int get continuousModeSteps => _continuousModeSteps;
+
+  /// Toggles the state of Dark Mode and notifies listeners.
+  void toggleDarkMode(bool value) {
+    _isDarkModeEnabled = value;
+    notifyListeners();
+    // TODO: Save to local storage or sync with the server
+  }
+
+  /// Toggles the state of Developer Mode and notifies listeners.
+  void toggleDeveloperMode(bool value) {
+    _isDeveloperModeEnabled = value;
+    notifyListeners();
+    // TODO: Save to local storage or sync with the server
+  }
+
+  /// Updates the state of Base URL and notifies listeners.
+  void updateBaseURL(String value) {
+    _baseURL = value;
+    notifyListeners();
+    // TODO: Save to local storage or sync with the server
+  }
+
+  /// Increments the number of Continuous Mode Steps and notifies listeners.
+  void incrementContinuousModeSteps() {
+    _continuousModeSteps += 1;
+    notifyListeners();
+    // TODO: Save to local storage or sync with the server
+  }
+
+  /// Decrements the number of Continuous Mode Steps and notifies listeners.
+  void decrementContinuousModeSteps() {
+    if (_continuousModeSteps > 1) {
+      // Ensure that the number of steps is at least 1
+      _continuousModeSteps -= 1;
+      notifyListeners();
+      // TODO: Save to local storage or sync with the server
+    }
+  }
+}