Add missing RewriteEngine On to .htaccess

The RewriteCond/RewriteRule directives for extension stripping were
present but RewriteEngine was never explicitly enabled. This relied on
a parent .htaccess having it on, which is a fragile assumption.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

.htaccess

@@ -2,6 +2,7 @@
 ServerSignature Off
 
 # remove php and html extensions
+RewriteEngine On
 RewriteCond %{REQUEST_FILENAME} !-f
 RewriteRule ^([^\.]+)$ $1.php [NC,L]
 RewriteRule ^([^\.]+)$ $1.html [NC,L]