Add theme persistence and comprehensive Android design guidelines

This commit adds persistent theme storage and comprehensive documentation for Android development.

Theme Persistence:
- Created ThemeStorage with platform-specific implementations (SharedPreferences/UserDefaults)
- Updated ThemeManager.initialize() to load saved theme on app start
- Integrated ThemeStorage initialization in MainActivity and MainViewController
- Theme selection now persists across app restarts

Documentation (CLAUDE.md):
- Added comprehensive Android Design System section
- Documented all 11 themes and theme management
- Provided color system guidelines (use MaterialTheme.colorScheme)
- Documented spacing system (AppSpacing/AppRadius constants)
- Added standard component usage examples (StandardCard, FormTextField, etc.)
- Included screen patterns (Scaffold, pull-to-refresh, lists)
- Provided button and dialog patterns
- Listed key design principles for Android development

Build Status:
- ✅ Android builds successfully
- ✅ iOS builds successfully
- ✅ Theme persistence works on both platforms

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

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

composeApp/src/iosMain/kotlin/com/example/mycrib/MainViewController.kt

@@ -5,6 +5,9 @@ import com.example.mycrib.storage.TokenManager
 import com.example.mycrib.storage.TokenStorage
 import com.example.mycrib.storage.TaskCacheManager
 import com.example.mycrib.storage.TaskCacheStorage
+import com.example.mycrib.storage.ThemeStorage
+import com.example.mycrib.storage.ThemeStorageManager
+import com.example.mycrib.ui.theme.ThemeManager
 
 fun MainViewController() = ComposeUIViewController {
     // Initialize TokenStorage with iOS TokenManager
@@ -13,5 +16,9 @@ fun MainViewController() = ComposeUIViewController {
     // Initialize TaskCacheStorage for offline task caching
     TaskCacheStorage.initialize(TaskCacheManager.getInstance())
 
+    // Initialize ThemeStorage and ThemeManager
+    ThemeStorage.initialize(ThemeStorageManager.getInstance())
+    ThemeManager.initialize()
+
     App()
 }

composeApp/src/iosMain/kotlin/com/example/mycrib/storage/ThemeStorageManager.ios.kt

@@ -0,0 +1,32 @@
+package com.example.mycrib.storage
+
+import platform.Foundation.NSUserDefaults
+
+/**
+ * iOS implementation of theme storage using NSUserDefaults.
+ */
+actual class ThemeStorageManager {
+    private val defaults = NSUserDefaults.standardUserDefaults
+
+    actual fun saveThemeId(themeId: String) {
+        defaults.setObject(themeId, forKey = KEY_THEME_ID)
+        defaults.synchronize()
+    }
+
+    actual fun getThemeId(): String? {
+        return defaults.stringForKey(KEY_THEME_ID)
+    }
+
+    actual fun clearThemeId() {
+        defaults.removeObjectForKey(KEY_THEME_ID)
+        defaults.synchronize()
+    }
+
+    companion object {
+        private const val KEY_THEME_ID = "theme_id"
+
+        private val instance by lazy { ThemeStorageManager() }
+
+        fun getInstance(): ThemeStorageManager = instance
+    }
+}