fix: update AppConstants.appVersion 1.2.2 → 1.5.1

The hardcoded app version constant was stuck at 1.2.2 while pubspec has
been through multiple releases up to 1.5.1+276. No callers depend on it
for functionality (only for display), but it drifts every release. Long
term this should be derived from package_info_plus.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: RedGridTactical

lib/core/constants/app_constants.dart

@@ -3,7 +3,7 @@ class AppConstants {
   AppConstants._();
 
   static const String appName = 'Red Grid Link';
-  static const String appVersion = '1.2.2';
+  static const String appVersion = '1.5.1';
 
   // Device limits
   static const int maxDevices = 8;