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/toc.php

@@ -1,5 +1,5 @@
 <nav class="toc">
-    <div class="light toc_label">table des matières</div>
+    <div class="light toc_label">sommaire</div>
     <ul>
         <?php foreach ($page->tocItems() as $item): ?>
             <li>