fix(css): correct justify-center syntax error in loading overlay

Fixed invalid CSS property 'justify-center;' on line 486 of
tractatus-theme.css. Changed to correct CSS property:
'justify-content: center;'

This was causing Firefox console errors:
"Unknown property 'justify-center;'. Declaration dropped."

The error was in the .loading-overlay class which is used for
loading states across admin pages. The invalid property prevented
proper centering of loading spinners on mobile devices.

Also regenerated minified CSS (39.4% size reduction: 24KB → 15KB).

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

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

public/css/tractatus-theme.css

@@ -483,7 +483,7 @@ h3 { letter-spacing: -0.015em; }
   backdrop-filter: blur(2px);
   display: flex;
   align-items: center;
-  justify-center;
+  justify-content: center;
   z-index: 50;
 }