Fix theme selection bug and update onboarding with AppTheme picker

- Change Theme enum from Int to String raw values to fix theme selection bug
- Replace OnboardingStyle icon/color pickers with unified AppTheme grid
- Remove visible text labels from voting layouts while keeping accessibility labels (WCAG 2.1 AA compliant)
- Update widget voting views to use icons only with proper accessibility support
- Consolidate app icons to single unified icon set

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

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

Shared/Models/Theme.swift

@@ -11,12 +11,23 @@ struct ThemeConstants {
     static let iconSize: CGFloat = 50
 }
 
-enum Theme: Int, CaseIterable {
-    case system
-    case iFeel
-    case dark
-    case light
-    
+enum Theme: String, CaseIterable {
+    case system = "system"
+    case iFeel = "iFeel"
+    case dark = "dark"
+    case light = "light"
+
+    // Legacy Int-based init for backwards compatibility with existing UserDefaults data
+    init?(legacyIntValue: Int) {
+        switch legacyIntValue {
+        case 0: self = .system
+        case 1: self = .iFeel
+        case 2: self = .dark
+        case 3: self = .light
+        default: return nil
+        }
+    }
+
     var title: String {
         switch self {
         case .system: