feat: add custom 404 error page

- Create stylish 404 page with glitch animation and CRT scanline effects
- Update nginx to serve custom 404 instead of falling back to index.html
- Page inherits site layout with WebGL background and audio player

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

nginx.conf

@@ -29,9 +29,12 @@ server {
         add_header Cache-Control "no-store, no-cache, must-revalidate";
     }
 
-    # Clean URLs - try files, then directories, then fallback to index.html
+    # Custom 404 page
+    error_page 404 /404.html;
+
+    # Clean URLs - try files, then directories, then 404
     location / {
-        try_files $uri $uri/ $uri.html /index.html;
+        try_files $uri $uri/ $uri.html =404;
     }
 
     # RSS feed