feat: Add appearance customization screen with background blur options

- Introduced `BackgroundBlurOption` data class and `BackgroundBlurPresets` object for managing background blur options.
- Created `AppearanceScreen` composable for selecting background colors and gradients, including a live preview of the selected option.
- Updated `OtherProfileScreen` and `ProfileScreen` to accept and utilize `backgroundBlurColorId` for consistent background blur across profiles.
- Enhanced `CollapsingOtherProfileHeader` and `CollapsingProfileHeader` to apply selected background blur options.

app/src/main/java/com/rosetta/messenger/data/PreferencesManager.kt

@@ -42,6 +42,9 @@ class PreferencesManager(private val context: Context) {
 
         // Language
         val APP_LANGUAGE = stringPreferencesKey("app_language") // "en", "ru", etc.
+
+        // Appearance / Customization
+        val BACKGROUND_BLUR_COLOR_ID = stringPreferencesKey("background_blur_color_id") // id from BackgroundBlurPresets
     }
 
     // ═════════════════════════════════════════════════════════════
@@ -189,4 +192,17 @@ class PreferencesManager(private val context: Context) {
     suspend fun setAppLanguage(value: String) {
         context.dataStore.edit { preferences -> preferences[APP_LANGUAGE] = value }
     }
+
+    // ═════════════════════════════════════════════════════════════
+    // 🎨 APPEARANCE / CUSTOMIZATION
+    // ═════════════════════════════════════════════════════════════
+
+    val backgroundBlurColorId: Flow<String> =
+            context.dataStore.data.map { preferences ->
+                preferences[BACKGROUND_BLUR_COLOR_ID] ?: "avatar" // Default: use avatar blur
+            }
+
+    suspend fun setBackgroundBlurColorId(value: String) {
+        context.dataStore.edit { preferences -> preferences[BACKGROUND_BLUR_COLOR_ID] = value }
+    }
 }