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

include/ui/game_screen.hpp

@@ -276,6 +276,15 @@ private:
     // Left-click targeting: distinguish click from camera drag
     glm::vec2 leftClickPressPos_ = glm::vec2(0.0f);
     bool leftClickWasPress_ = false;
+
+    // Level-up ding animation
+    static constexpr float DING_DURATION = 3.0f;
+    float dingTimer_ = 0.0f;
+    uint32_t dingLevel_ = 0;
+    void renderDingEffect();
+
+public:
+    void triggerDing(uint32_t newLevel);
 };
 
 } // namespace ui