fix: use npx to run pa11y if not globally installed in check_wcag.py

Co-authored-by: aider (o3) <aider@aider.chat>

check_wcag.py

@@ -23,6 +23,7 @@ import csv
 import json
 import subprocess
 import sys
+import shutil
 from pathlib import Path
 from typing import Dict, List, Set
 
@@ -37,8 +38,9 @@ def run_pa11y(url: str) -> List[dict]:
     Returns an empty list if pa11y fails.
     """
     try:
+        cmd_base = ["pa11y"] if shutil.which("pa11y") else ["npx", "pa11y"]
         result = subprocess.run(
-            ["pa11y", url, "--standard", "WCAG2AA", "--reporter", "json"],
+            [*cmd_base, url, "--standard", "WCAG2AA", "--reporter", "json"],
             capture_output=True,
             text=True,
             check=False,