Add theme picker with 7 color schemes

- Teal: Cool cyan and teal tones (default)
- Orbit: Bold navy and orange
- Retro: Classic columbia blue
- Clutch: Championship red and gold
- Monochrome: Clean grayscale aesthetic
- Sunset: Warm oranges and purples
- Midnight: Deep blues and gold

Features:
- Theme selection persisted via UserDefaults
- ThemeManager singleton for app-wide theme state
- All Theme colors now dynamically switch based on selection
- Settings UI shows color preview circles for each theme

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

SportsTime/Features/Settings/ViewModels/SettingsViewModel.swift

@@ -12,6 +12,12 @@ final class SettingsViewModel {
 
     // MARK: - User Preferences (persisted via UserDefaults)
 
+    var selectedTheme: AppTheme {
+        didSet {
+            ThemeManager.shared.currentTheme = selectedTheme
+        }
+    }
+
     var selectedSports: Set<Sport> {
         didSet { savePreferences() }
     }
@@ -41,6 +47,9 @@ final class SettingsViewModel {
         // Load from UserDefaults using local variables first
         let defaults = UserDefaults.standard
 
+        // Theme
+        self.selectedTheme = ThemeManager.shared.currentTheme
+
         // Selected sports
         if let sportStrings = defaults.stringArray(forKey: "selectedSports") {
             self.selectedSports = Set(sportStrings.compactMap { Sport(rawValue: $0) })
@@ -93,6 +102,7 @@ final class SettingsViewModel {
     }
 
     func resetToDefaults() {
+        selectedTheme = .teal
         selectedSports = Set(Sport.supported)
         maxDrivingHoursPerDay = 8
         maxTripOptions = 10