Fix language persistence for guide link and navbar brand

- Add lang parameter to guide link in footer template
- Add lang parameter to navbar brand link in header template
- Ensures language selection persists when clicking logo or guide icon
- Completes language persistence across all navigation elements

engine/templates/assets/header.mustache

@@ -1,6 +1,6 @@
 <header class="navbar navbar-expand-lg navbar-dark" style="background-color: var(--header-bg);">
     <div class="container-fluid">
-        <a class="navbar-brand" href="?page={{default_page}}">
+        <a class="navbar-brand" href="?page={{default_page}}&lang={{current_lang}}">
             <img src="/assets/icon.svg" alt="CodePress Logo" width="32" height="32" class="me-2">
             {{site_title}}
         </a>