sync: fallback to PAT_TOKEN secret when override-token is empty

Explain briefly:
UI input field doesn’t evaluate ${{ secrets.* }}, users may paste it and it becomes a literal string.
Add fallback to secrets.PAT_TOKEN to make workflow_dispatch easier and safer.

.github/workflows/sync.yaml

@@ -13,7 +13,7 @@ on:
         required: true
         default: false
       override-token:
-        description: '[Optional] Replace the default token with the supplied PAT. Used to resolve `refusing to allow a GitHub App to create or update workflow... `'
+        description: '[Optional] Paste a PAT here. If left blank, PAT_TOKEN secret will be used (if set), otherwise GITHUB_TOKEN.'
         required: false
 
 jobs: