feat(02-01): create SettingsProvider for theme and unit state

- Created lib/providers/settings_provider.dart
- Extends ChangeNotifier for reactive state management
- Constructor accepts pre-initialized SharedPreferences instance
- Loads theme index and speed unit synchronously from SharedPreferences
- cycleTheme() and cycleUnit() methods with haptic feedback
- Persists settings to SharedPreferences on every change
- Both cycle methods call notifyListeners() for reactive UI updates
- No notifyListeners() in constructor (runs before first build)

lib/providers/settings_provider.dart

@@ -0,0 +1,50 @@
+import 'package:flutter/foundation.dart';
+import 'package:flutter/services.dart';
+import 'package:shared_preferences/shared_preferences.dart';
+import '../config/speed_unit.dart';
+import '../config/color_themes.dart';
+import '../services/logger.dart';
+
+class SettingsProvider extends ChangeNotifier {
+  final SharedPreferences _prefs;
+
+  int _currentThemeIndex = 0;
+  SpeedUnit _currentUnit = SpeedUnit.kmh;
+
+  // Getters for read access
+  int get currentThemeIndex => _currentThemeIndex;
+  SpeedUnit get currentUnit => _currentUnit;
+  ColorTheme get currentTheme => ColorThemes.getTheme(_currentThemeIndex);
+
+  /// Constructor loads settings synchronously from pre-initialized SharedPreferences.
+  /// SharedPreferences MUST be initialized before constructing this provider.
+  SettingsProvider(this._prefs) {
+    _loadSettings();
+    Logger.info('Settings loaded: theme=$_currentThemeIndex, unit=${_currentUnit.label}', 'SettingsProvider');
+  }
+
+  void _loadSettings() {
+    _currentThemeIndex = _prefs.getInt('themeIndex') ?? 0;
+    final unitIndex = _prefs.getInt('unitIndex') ?? 0;
+    _currentUnit = SpeedUnit.values[unitIndex.clamp(0, SpeedUnit.values.length - 1)];
+    // No notifyListeners() — constructor runs before first build
+  }
+
+  /// Cycle to next theme with haptic feedback and persistence
+  void cycleTheme() {
+    HapticFeedback.lightImpact();
+    _currentThemeIndex = ColorThemes.getNextThemeIndex(_currentThemeIndex);
+    _prefs.setInt('themeIndex', _currentThemeIndex);
+    Logger.debug('Theme cycled to index $_currentThemeIndex', 'SettingsProvider');
+    notifyListeners();
+  }
+
+  /// Cycle to next speed unit with haptic feedback and persistence
+  void cycleUnit() {
+    HapticFeedback.lightImpact();
+    _currentUnit = _currentUnit.next;
+    _prefs.setInt('unitIndex', _currentUnit.index);
+    Logger.debug('Unit cycled to ${_currentUnit.label}', 'SettingsProvider');
+    notifyListeners();
+  }
+}