refactor(i18n): share supported language choices

Use one non-generated language catalog in setup and appearance settings. Filter supported locale codes once and retain the resulting immutable picker list.

lib/constants/language_choices.dart

@@ -0,0 +1,33 @@
+import 'package:flutter/material.dart';
+import 'package:spotiflac_android/l10n/supported_locales.dart';
+
+/// Display choices shared by onboarding and appearance settings.
+const appLanguageChoices = [
+  ('system', 'System Default', Icons.phone_android),
+  ('en', 'English', Icons.language),
+  ('ar', 'العربية', Icons.language),
+  ('id', 'Bahasa Indonesia', Icons.language),
+  ('de', 'Deutsch', Icons.language),
+  ('es', 'Español', Icons.language),
+  ('es_ES', 'Español (España)', Icons.language),
+  ('fr', 'Français', Icons.language),
+  ('hi', 'हिन्दी', Icons.language),
+  ('it', 'Italiano', Icons.language),
+  ('ja', '日本語', Icons.language),
+  ('ko', '한국어', Icons.language),
+  ('nl', 'Nederlands', Icons.language),
+  ('pt', 'Português', Icons.language),
+  ('pt_PT', 'Português (Brasil)', Icons.language),
+  ('ru', 'Русский', Icons.language),
+  ('tr', 'Türkçe', Icons.language),
+  ('uk', 'Українська', Icons.language),
+  ('zh_CN', '简体中文', Icons.language),
+  ('zh_TW', '繁體中文', Icons.language),
+];
+
+final supportedLanguageChoices = List<(String, String, IconData)>.unmodifiable(
+  appLanguageChoices.where(
+    (choice) =>
+        choice.$1 == 'system' || filteredLocaleCodes.contains(choice.$1),
+  ),
+);