Refactor: Replace sidebar with horizontal navigation bar

- Remove sidebar and toggle functionality
- Add Bootstrap navbar with dropdown menus
- Move navigation to top between header and content
- Update menu rendering for Bootstrap dropdowns
- Clean up unused files (header.mustache, sidebar.mustache, sidebar.js)
- Add guide link with book icon in footer
- Simplify layout structure
- Remove duplicate code and fix syntax errors
- Add .gitignore for node_modules and other temp files

engine/templates/assets/navigation.mustache

@@ -0,0 +1,23 @@
+<nav class="navbar navbar-expand-lg navbar-dark bg-primary">
+    <div class="container-fluid">
+        <a class="navbar-brand" href="?page={{default_page}}">
+            <img src="/assets/icon.svg" alt="CodePress Logo" width="32" height="32" class="me-2">
+            {{site_title}}
+        </a>
+        
+        <button class="navbar-toggler" type="button" data-bs-toggle="collapse" data-bs-target="#navbarNav" aria-controls="navbarNav" aria-expanded="false" aria-label="Toggle navigation">
+            <span class="navbar-toggler-icon"></span>
+        </button>
+        
+        <div class="collapse navbar-collapse" id="navbarNav">
+            <ul class="navbar-nav me-auto">
+                {{{menu}}}
+            </ul>
+            
+            <form class="d-flex" method="GET" action="">
+                <input class="form-control me-2" type="search" name="search" placeholder="Search..." value="{{search_query}}">
+                <button class="btn btn-outline-light" type="submit">Search</button>
+            </form>
+        </div>
+    </div>
+</nav>