Add Mots-clés and Langues management to contenus page

- Add searchLanguages, getAllLanguagesWithCount, renameLanguage, mergeLanguage, deleteLanguage to Database
- Create actions/language.php handler with rename/merge/merge_bulk/delete actions
- Add merge_bulk action to actions/tag.php
- Add Mots-clés section to contenus template with HTMX search, select checkboxes, rename/delete/merge buttons, and multi-select merge toolbar
- Add Langues section to contenus template with same pattern
- Create contenus-tags-fragment.php and contenus-languages-fragment.php HTMX fragments
- Remove form-settings- from flat-fieldset CSS selector so fieldsets in contenus retain border/padding
- contenus.php: add 'Gérer les mots-clés' link to /admin/tags.php
- contenus.php: add Langues fieldset with HTMX search + table (rename/merge/delete/bulk)
- tags.php: add HTMX search bar, checkbox column, bulk merge toolbar
- Create tags-fragment.php and contenus-langues-fragment.php for HTMX
- Remove tab component and associated CSS
- Simplify JS: separate tags/langues-prefixed functions
- Fix redirects: tag.php defaults to /admin/tags.php, supports return override
- Keep tags.php standalone page and Mots-clés button unchanged

app/templates/admin/parametres.php

@@ -1,3 +1,5 @@
+<div class="admin-with-toc">
+<?php include APP_ROOT . '/templates/admin/partials/admin-toc.php'; ?>
 <main id="main-content">
     <h1>Paramètres</h1>
 
@@ -458,6 +460,7 @@
         </div>
     </section>
 </main>
+</div>
 
 <script>
 function copyLogContent(btn) {