Clear game handler DBC caches on expansion switch

Add resetDbcCaches() to GameHandler to clear stale spell name, skill,
taxi, and talent caches when switching expansions. Called from
reloadExpansionData() so switching servers (e.g. WotLK to Turtle)
reloads DBC data instead of using stale entries from the previous
expansion.

src/core/application.cpp

@@ -516,6 +516,11 @@ void Application::reloadExpansionData() {
     // Reset map name cache so it reloads from new expansion's Map.dbc
     mapNameCacheLoaded_ = false;
     mapNameById_.clear();
+
+    // Reset game handler DBC caches so they reload from new expansion data
+    if (gameHandler) {
+        gameHandler->resetDbcCaches();
+    }
 }
 
 void Application::logoutToLogin() {