Add sidebar TOC, simplify Données Secondaires section

- Rename 'Éditer Données Secondaires' → 'Données Secondaires', remove fieldset wrapper on Mots-clés link
- Create admin-toc.php partial: IntersectionObserver-based sidebar nav
- Include TOC on contenus.php, acces.php, parametres.php
- Add .admin-with-toc flex layout (sidebar + main) and .admin-toc CSS
- Fonts (Ductus, BBB DM Sans): verified loaded via variables.css → common.css import chain
- TOC: move inside <main> as <aside>, content in <article>, fix scrolling
- Lazy load: hx-trigger='load delay:100ms' with spinner (htmx-indicator) for tags/langues
- Inline rename: edit button in Nom cell, HTMX post for rename, validate+ cancel buttons
- Checkbox column: width:1% / fit-content
- Remove per-row merge forms/selects, only bulk merge when ≥2 checkboxes selected
- Remove per-row merge dialogs, keep only bulk merge and delete dialogs
- Add htmx-settling CSS transition for lazy-load fade-in
- Update acces.php/parametres.php: article layout, TOC inside main
- TOC: DOMContentLoaded guard, use <nav>+<a> directly instead of <ul>/<li>
- Section spacing: margin-bottom on sections and fieldsets in admin-main--toc
- Language dedup: GROUP BY LOWER(name) in getAllLanguagesWithCount and searchLanguages
- deduplicateLanguages() merges duplicate names and reassigns thesis_languages
- Sticky bulk-actions: position:sticky;top:0;z-index:10
- Tags toolbar: title left, stat count right (margin-left:auto), search bar under
- Tags count stat updated via hx-swap-oob from fragment
- Remove margin/max-width from .admin-main--toc
- Gap between TOC and article: --space-xs, sticky top: --space-xs
- Main padding: --space-s / --space-m / --space-xl (was --space-l/--space-l/--space-2xl)
- Article padding-top: --space-m

app/templates/admin/parametres.php

@@ -1,6 +1,7 @@
-<div class="admin-with-toc">
-<?php include APP_ROOT . '/templates/admin/partials/admin-toc.php'; ?>
-<main id="main-content">
+<main id="main-content" class="admin-main--toc">
+    <?php include APP_ROOT . '/templates/admin/partials/admin-toc.php'; ?>
+
+    <article>
     <h1>Paramètres</h1>
 
     <!-- ══════════════════════════════════════════════════════════════
@@ -459,8 +460,8 @@
             <?php endif; ?>
         </div>
     </section>
+    </article>
 </main>
-</div>
 
 <script>
 function copyLogContent(btn) {