refactor: use absolute path for clear command (brisbanesocialchess#640)

This PR refactors the way the clear screen command is invoked to address a security audit finding. By specifying the full executable path, we eliminate the risk of running an unintended binary and improve consistency across environments.

- Audit: Starting a process with a partial executable path
  The code previously used os.system("clear"), which relies on the user’s $PATH and can be hijacked by a malicious executable. This change replaces it with os.system("/usr/bin/clear"), ensuring that the standard system utility is executed and mitigating potential security risks.

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Author: deepsource-autofix[bot]

scripts/chess_animation.py

@@ -28,7 +28,7 @@ def clear_screen():
         if os.name == "nt":
             os.system("cls")  # Windows
         else:
-            os.system("clear")  # macOS/Linux
+            os.system("/usr/bin/clear")  # macOS/Linux
     except Exception:
         print("\033[H\033[J", end="")  # ANSI escape sequence fallback