side-panel : refactor nav + toc into single side-panel with view switching

Rename nav.php to side-panel.php to host both navigation and table of contents
views. The panel uses data-view attributes to switch between nav and toc content.
Footer buttons updated to target the unified panel. TOC button now visible on desktop.

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

site/snippets/header.php

@@ -100,8 +100,5 @@ $entryTopPos ??= 20;
       </h1>
     </a>
   </header>
-  <?php snippet('nav') ?>
-  <?php if ($page->hasToc()): ?>
-    <?php snippet('panel-toc') ?>
-  <?php endif ?>
+  <?php snippet('side-panel') ?>
   <div id="nav-overlay"></div>