ci: update preview URL to xmoj-docs.infinityfreeapp.com

Agent-Logs-Url: https://github.com/XMOJ-Script-dev/docs/sessions/6c49dfe8-b721-45ea-9c4a-187134ec1c5a

Co-authored-by: PythonSmall-Q <106425289+PythonSmall-Q@users.noreply.github.com>

Author: Copilot

.github/workflows/preview.yml

@@ -43,7 +43,7 @@ jobs:
         with:
           script: |
             const prNumber = context.payload.pull_request.number;
-            const previewUrl = `http://ftpupload.net/pr-${prNumber}/`;
+            const previewUrl = `http://xmoj-docs.infinityfreeapp.com/pr-${prNumber}/`;
             const runUrl = `https://github.com/${context.repo.owner}/${context.repo.repo}/actions/runs/${context.runId}`;
             const body = [
               '## 📖 文档预览',