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/commonMain/kotlin/com/example/mycrib/storage/ThemeStorage.kt

@@ -0,0 +1,35 @@
+package com.example.mycrib.storage
+
+/**
+ * Cross-platform theme storage for persisting theme selection.
+ * Uses platform-specific implementations (SharedPreferences on Android, UserDefaults on iOS).
+ */
+object ThemeStorage {
+    private var manager: ThemeStorageManager? = null
+
+    fun initialize(themeManager: ThemeStorageManager) {
+        manager = themeManager
+    }
+
+    fun saveThemeId(themeId: String) {
+        manager?.saveThemeId(themeId)
+    }
+
+    fun getThemeId(): String? {
+        return manager?.getThemeId()
+    }
+
+    fun clearThemeId() {
+        manager?.clearThemeId()
+    }
+}
+
+/**
+ * Platform-specific theme storage interface.
+ * Each platform implements this using their native storage mechanisms.
+ */
+expect class ThemeStorageManager {
+    fun saveThemeId(themeId: String)
+    fun getThemeId(): String?
+    fun clearThemeId()
+}

composeApp/src/commonMain/kotlin/com/example/mycrib/ui/theme/ThemeManager.kt

@@ -3,11 +3,11 @@ package com.example.mycrib.ui.theme
 import androidx.compose.runtime.getValue
 import androidx.compose.runtime.mutableStateOf
 import androidx.compose.runtime.setValue
+import com.example.mycrib.storage.ThemeStorage
 
 /**
  * ThemeManager - Singleton for managing app themes
- * Matches iOS ThemeManager functionality
- * TODO: Add DataStore persistence
+ * Matches iOS ThemeManager functionality with persistent storage
  */
 object ThemeManager {
     private const val DEFAULT_THEME_ID = "default"
@@ -19,16 +19,28 @@ object ThemeManager {
         private set
 
     /**
-     * Set theme by ID
+     * Initialize theme manager and load saved theme
+     * Call this after ThemeStorage.initialize()
+     */
+    fun initialize() {
+        val savedThemeId = ThemeStorage.getThemeId()
+        if (savedThemeId != null) {
+            val savedTheme = AppThemes.getThemeById(savedThemeId)
+            currentTheme = savedTheme
+        }
+    }
+
+    /**
+     * Set theme by ID and persist the selection
      */
     fun setTheme(themeId: String) {
         val newTheme = AppThemes.getThemeById(themeId)
         currentTheme = newTheme
-        // TODO: Persist theme selection
+        ThemeStorage.saveThemeId(themeId)
     }
 
     /**
-     * Set theme by ThemeColors object
+     * Set theme by ThemeColors object and persist the selection
      */
     fun setTheme(theme: ThemeColors) {
         setTheme(theme.id)
@@ -42,7 +54,7 @@ object ThemeManager {
     }
 
     /**
-     * Reset to default theme
+     * Reset to default theme and persist the selection
      */
     fun resetToDefault() {
         setTheme(DEFAULT_THEME_ID)