feat(localization): add Chinese (zh) language support

Clean-branch integration of community PR #300 (ADAIBLOG) against current
develop. Adds all 423 translation keys, resolves the 15 keys missing from
the original PR (13 QR/share-meal keys + 2 American football PA keys), and
wires up all integration points.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

Author: jordan-lee-code

lib/core/utils/supported_language.dart

@@ -1,7 +1,8 @@
 enum SupportedLanguage {
   en,
   pl,
-  de;
+  de,
+  zh;
 
   factory SupportedLanguage.fromCode(String localeCode) {
     final languageCode = localeCode.split('_').first;
@@ -12,6 +13,8 @@ enum SupportedLanguage {
         return SupportedLanguage.de;
       case 'pl':
         return SupportedLanguage.pl;
+      case 'zh':
+        return SupportedLanguage.zh;
       default:
         return SupportedLanguage.en;
     }

lib/features/add_meal/data/dto/fdc_sp/sp_const.dart

@@ -28,6 +28,8 @@ class SPConst {
         return fdcFoodDescriptionDe;
       case SupportedLanguage.pl:
         return fdcFoodDescriptionEn;
+      case SupportedLanguage.zh:
+        return fdcFoodDescriptionEn;
     }
   }
 }

lib/features/add_meal/data/dto/fdc_sp/sp_fdc_food_dto.dart

@@ -31,6 +31,8 @@ class SpFdcFoodDTO {
         return descriptionDe;
       case SupportedLanguage.pl:
         return descriptionEn;
+      case SupportedLanguage.zh:
+        return descriptionEn;
     }
   }
 

lib/features/add_meal/data/dto/off/off_product_dto.dart

@@ -43,6 +43,9 @@ class OFFProductDTO {
       case SupportedLanguage.pl:
         localeName = product_name;
         break;
+      case SupportedLanguage.zh:
+        localeName = product_name;
+        break;
     }
 
     // If local language is not available, return available language

lib/generated/intl/messages_all.dart

@@ -23,6 +23,7 @@ import 'messages_it.dart' as messages_it;
 import 'messages_pl.dart' as messages_pl;
 import 'messages_tr.dart' as messages_tr;
 import 'messages_uk.dart' as messages_uk;
+import 'messages_zh.dart' as messages_zh;
 
 typedef Future<dynamic> LibraryLoader();
 Map<String, LibraryLoader> _deferredLibraries = {
@@ -33,6 +34,7 @@ Map<String, LibraryLoader> _deferredLibraries = {
   'pl': () => new SynchronousFuture(null),
   'tr': () => new SynchronousFuture(null),
   'uk': () => new SynchronousFuture(null),
+  'zh': () => new SynchronousFuture(null),
 };
 
 MessageLookupByLibrary? _findExact(String localeName) {
@@ -51,6 +53,8 @@ MessageLookupByLibrary? _findExact(String localeName) {
       return messages_tr.messages;
     case 'uk':
       return messages_uk.messages;
+    case 'zh':
+      return messages_zh.messages;
     default:
       return null;
   }