Replace apropos/licence/charte sticky nav with responsive details/summary TOC

- Convert .apropos-toc <nav> to <details class="toc" open> in all three templates
- Add caret-down icon to summary (visible only on mobile via media queries)
- Desktop (≥768px): sticky sidebar via CSS grid, force-open via pointer-events:none, hide caret
- Mobile (≤767px): single column, collapsible TOC with rotating caret, margin-top below search bar
- Rename .apropos-toc-link to .toc-sidebar-link
- Merge 900px and 600px breakpoints into single 600px mobile media query
- Rename apropos.css → content-page.css; update 3 controller references

app/templates/public/licence.php

@@ -1,15 +1,15 @@
 <main class="page-content" id="main-content">
 
-    <!-- LEFT: sticky table of contents -->
+    <!-- Table of contents: collapsible on mobile, force-open on desktop -->
     <?php if (!empty($tocItems)): ?>
-    <nav class="apropos-toc" aria-label="Sections de la page">
-        <p class="apropos-toc-label">PARTIES</p>
+    <details class="toc" open>
+        <summary><?= icon('caret-down', 0, 'toc-caret') ?> PARTIES</summary>
         <ul>
             <?php foreach ($tocItems as $item): ?>
             <li><a href="<?= htmlspecialchars($item['href']) ?>"><?= htmlspecialchars($item['label']) ?></a></li>
             <?php endforeach; ?>
         </ul>
-    </nav>
+    </details>
     <?php endif; ?>
 
     <div class="content">