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/partials/admin-toc.php

@@ -0,0 +1,78 @@
+<?php
+/**
+ * admin-toc.php — sidebar table-of-contents for long admin pages.
+ *
+ * Scans <section aria-labelledby="..."> elements in #main-content and builds a
+ * slim vertical nav. Uses IntersectionObserver to highlight the active section.
+ *
+ * Usage: include APP_ROOT . '/templates/admin/partials/admin-toc.php';
+ */
+?>
+<nav id="admin-toc" class="admin-toc" aria-label="Sur cette page">
+    <ul class="admin-toc-list" id="admin-toc-list">
+        <!-- populated by JS -->
+    </ul>
+</nav>
+
+<script>
+(function() {
+    var main = document.getElementById('main-content');
+    if (!main) return;
+
+    var tocList = document.getElementById('admin-toc-list');
+    if (!tocList) return;
+
+    // Find all labelled sections
+    var sections = main.querySelectorAll('section[aria-labelledby]');
+    if (sections.length < 2) {
+        document.getElementById('admin-toc').style.display = 'none';
+        return;
+    }
+
+    var items = [];
+
+    sections.forEach(function(sec) {
+        var headingId = sec.getAttribute('aria-labelledby');
+        var heading = document.getElementById(headingId);
+        if (!heading) return;
+
+        var li = document.createElement('li');
+        var a = document.createElement('a');
+        a.href = '#' + sec.id;
+        a.textContent = heading.textContent;
+        a.setAttribute('data-toc-target', sec.id);
+        li.appendChild(a);
+        tocList.appendChild(li);
+
+        // Ensure section has an id for anchoring
+        if (!sec.id) {
+            sec.id = headingId;
+        }
+
+        items.push({ section: sec, link: a });
+    });
+
+    // IntersectionObserver: highlight the link whose section is most visible
+    var observer = new IntersectionObserver(function(entries) {
+        var best = null;
+        var bestRatio = 0;
+        entries.forEach(function(e) {
+            if (e.intersectionRatio > bestRatio) {
+                bestRatio = e.intersectionRatio;
+                best = e.target;
+            }
+        });
+        if (best) {
+            items.forEach(function(item) {
+                var isActive = item.section === best;
+                item.link.classList.toggle('admin-toc-active', isActive);
+            });
+        }
+    }, {
+        rootMargin: '-10% 0px -70% 0px',
+        threshold: [0, 0.25, 0.5, 0.75, 1]
+    });
+
+    items.forEach(function(item) { observer.observe(item.section); });
+})();
+</script>