feat: focus target frame with health/power bars and cast bar

Add a compact focus target frame on the right side of the screen
when the player has a focus target set via /focus.

- Shows [Focus] label, name (colored by hostility/level diff), level
- HP bar with green→yellow→red coloring; power bar with type colors
- Cast bar showing spell name and remaining time when focus is casting
- Clicking the frame targets the focus entity
- Clears automatically when focus is lost (/clearfocus)

include/ui/game_screen.hpp

@@ -189,6 +189,7 @@ private:
      * Render target frame
      */
     void renderTargetFrame(game::GameHandler& gameHandler);
+    void renderFocusFrame(game::GameHandler& gameHandler);
 
     /**
      * Render pet frame (below player frame when player has an active pet)