feat(apps): add Stirling PDF manifest

[ArqamQazi]

Summary

Added stirling pdf.

Checklist

• I did not include changes under .generated/.
• I updated tests or documentation when behavior changed.
• I carefully reviewed any publisher, sandbox, trust-root, or workflow changes.

Application checklist

• [ x This pull request adds only apps/stirling-pdf.json.
• I used the application template and verified every source and identifier.
• The latest stable release provides at least one working AppImage.
• The sandbox policy grants only the access the application needs.
• I will maintain the manifest when its sources, permissions, or availability change.