Fix locale picker navigation on step.html pages

The JS was using index.html-specific path logic that broke on step.html.
Now uses lastIndexOf('/') to correctly insert locale before the filename:
/repo/step.html → /repo/es/step.html

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: brunoborges

docs/step.html

@@ -369,7 +369,7 @@ <h1>Error Loading Content</h1>
           if(t===locale){close();return;}
           var p=location.pathname;
           if(locale!=='en')p=p.replace(new RegExp('/'+locale+'/'),'/');
-          if(t!=='en'){var base=p.replace(/\/index\.html$/,'/');if(!base.endsWith('/'))base+='/';p=base+t+'/';}
+          if(t!=='en'){var ls=p.lastIndexOf('/');p=p.substring(0,ls+1)+t+p.substring(ls);}
           localStorage.setItem('preferred-locale',t);
           window.location.href=p+location.search;
         });