Add level-up ding animation with cheer emote and test button

- Trigger ding when UNIT_FIELD_LEVEL increases for player: plays LevelUp sound,
  cheer emote animation, and shows screen overlay
- renderDingEffect(): 3 expanding golden rings + "LEVEL X!" / "DING!" text
  drawn via ImDrawList foreground (3s duration, fades out last 0.8s)
- triggerDing() wires sound (UiSoundManager::playLevelUp) + emote + overlay
- LevelUpCallback in GameHandler fires on genuine level increase (newLevel > oldLevel)
- "Test: Level Up" button in escape menu triggers ding at current player level

src/core/application.cpp

@@ -1280,6 +1280,13 @@ void Application::setupUICallbacks() {
         despawnOnlineGameObject(guid);
     });
 
+    // Level-up callback — play sound, cheer emote, and trigger UI ding overlay
+    gameHandler->setLevelUpCallback([this](uint32_t newLevel) {
+        if (uiManager) {
+            uiManager->getGameScreen().triggerDing(newLevel);
+        }
+    });
+
     // Mount callback (online mode) - defer heavy model load to next frame
     gameHandler->setMountCallback([this](uint32_t mountDisplayId) {
         if (mountDisplayId == 0) {