Add pet frame UI below player frame

Shows active pet name, level, health bar, and power bar (mana/focus/rage/energy)
when the player has an active pet. Clicking the pet name targets it. A Dismiss
button sends CMSG_PET_ACTION to dismiss the pet. Frame uses green border to
visually distinguish it from the player/target frames.

include/ui/game_screen.hpp

@@ -182,6 +182,11 @@ private:
      */
     void renderTargetFrame(game::GameHandler& gameHandler);
 
+    /**
+     * Render pet frame (below player frame when player has an active pet)
+     */
+    void renderPetFrame(game::GameHandler& gameHandler);
+
     /**
      * Process targeting input (Tab, Escape, click)
      */