Fix single post layout to fill available width

Removed the restrictive max-width constraint on .single-post that was
causing the content to appear narrow with excessive whitespace. Changed
from max-width: 40em to max-width: 100% so the content properly fills
the grid column, matching the reference layout at philschmid.de.

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

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

Author: jack-arturo

assets/css/custom.css

@@ -1531,7 +1531,7 @@ button {
 
 /* Center: Main content */
 .single-post {
-  max-width: 40em;
+  max-width: 100%;
 }
 
 .single-post .entry-header {