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/androidMain/kotlin/com/example/mycrib/MainActivity.kt

@@ -26,6 +26,9 @@ 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
 import com.example.mycrib.fcm.FCMManager
 import kotlinx.coroutines.launch
 
@@ -42,6 +45,10 @@ class MainActivity : ComponentActivity(), SingletonImageLoader.Factory {
         // Initialize TaskCacheStorage for offline task caching
         TaskCacheStorage.initialize(TaskCacheManager.getInstance(applicationContext))
 
+        // Initialize ThemeStorage and ThemeManager
+        ThemeStorage.initialize(ThemeStorageManager.getInstance(applicationContext))
+        ThemeManager.initialize()
+
         // Handle deep link from intent
         handleDeepLink(intent)
 

composeApp/src/androidMain/kotlin/com/example/mycrib/storage/ThemeStorageManager.android.kt

@@ -0,0 +1,37 @@
+package com.example.mycrib.storage
+
+import android.content.Context
+import android.content.SharedPreferences
+
+/**
+ * Android implementation of theme storage using SharedPreferences.
+ */
+actual class ThemeStorageManager(context: Context) {
+    private val prefs: SharedPreferences = context.getSharedPreferences(PREFS_NAME, Context.MODE_PRIVATE)
+
+    actual fun saveThemeId(themeId: String) {
+        prefs.edit().putString(KEY_THEME_ID, themeId).apply()
+    }
+
+    actual fun getThemeId(): String? {
+        return prefs.getString(KEY_THEME_ID, null)
+    }
+
+    actual fun clearThemeId() {
+        prefs.edit().remove(KEY_THEME_ID).apply()
+    }
+
+    companion object {
+        private const val PREFS_NAME = "mycrib_theme_prefs"
+        private const val KEY_THEME_ID = "theme_id"
+
+        @Volatile
+        private var instance: ThemeStorageManager? = null
+
+        fun getInstance(context: Context): ThemeStorageManager {
+            return instance ?: synchronized(this) {
+                instance ?: ThemeStorageManager(context.applicationContext).also { instance = it }
+            }
+        }
+    }
+}