chore: clean up content and assets

- Remove unused custom.css from assets
- Update sidebar-slash-spec.png image
- Clean up front matter formatting in resources post (remove unnecessary quotes)

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

Author: tonykay

assets/css/custom.css

@@ -1,9 +1,9 @@
 ---
 title: "Resources - Under Construction"
-date: "2025-10-31"
+date: 2025-11-01
 linktitle: resources-under-construction
 slug: resources-under-construction
-thumbnail: "../../images/under-construction.png"
+thumbnail: ../../images/under-construction.png
 weight: 10
 categories:
   - Resources