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/public/admin/actions/language.php

@@ -0,0 +1,72 @@
+<?php
+require_once __DIR__ . '/../../../bootstrap.php';
+require_once __DIR__ . '/../../../src/AdminAuth.php';
+AdminAuth::requireLogin();
+
+if ($_SERVER['REQUEST_METHOD'] !== 'POST'
+    || !isset($_POST['csrf_token'], $_SESSION['csrf_token'])
+    || !hash_equals($_SESSION['csrf_token'], $_POST['csrf_token'])) {
+    http_response_code(403);
+    die("Accès refusé.");
+}
+
+require_once __DIR__ . '/../../../src/Database.php';
+require_once __DIR__ . '/../../../src/AdminLogger.php';
+require_once __DIR__ . '/../../../src/ErrorHandler.php';
+
+try {
+    $db     = new Database();
+    $logger = AdminLogger::make();
+    $action = $_POST['action'] ?? '';
+
+    switch ($action) {
+        case 'rename':
+            $id      = filter_var($_POST['language_id'] ?? '', FILTER_VALIDATE_INT);
+            $newName = trim($_POST['new_name'] ?? '');
+            if (!$id || $newName === '') throw new Exception("Paramètres invalides.");
+            $db->renameLanguage($id, $newName);
+            break;
+
+        case 'merge':
+            $sourceId = filter_var($_POST['source_id'] ?? '', FILTER_VALIDATE_INT);
+            $targetId = filter_var($_POST['target_id'] ?? '', FILTER_VALIDATE_INT);
+            if (!$sourceId || !$targetId) throw new Exception("Paramètres invalides.");
+            $db->mergeLanguage($sourceId, $targetId);
+            break;
+
+        case 'merge_bulk':
+            $targetId = filter_var($_POST['target_id'] ?? '', FILTER_VALIDATE_INT);
+            $sourceIds = isset($_POST['selected_langs']) && is_array($_POST['selected_langs'])
+                ? array_map('intval', $_POST['selected_langs'])
+                : [];
+            if (!$targetId || empty($sourceIds)) throw new Exception("Paramètres invalides.");
+            $sourceIds = array_values(array_diff($sourceIds, [$targetId]));
+            if (empty($sourceIds)) throw new Exception("Aucune source à fusionner.");
+            foreach ($sourceIds as $sid) {
+                $db->mergeLanguage($sid, $targetId);
+            }
+            break;
+
+        case 'delete':
+            $id = filter_var($_POST['language_id'] ?? '', FILTER_VALIDATE_INT);
+            if (!$id) throw new Exception("ID invalide.");
+            $db->deleteLanguage($id);
+            break;
+
+        default:
+            throw new Exception("Action inconnue.");
+    }
+
+    App::flash('success', "Opération effectuée.");
+} catch (Exception $e) {
+    ErrorHandler::log('language', $e);
+    App::flash('error', ErrorHandler::userMessage($e));
+}
+
+$redirect = '/admin/contenus.php';
+// Allow the caller to override the redirect
+if (!empty($_POST['return']) && str_starts_with($_POST['return'], '/')) {
+    $redirect = $_POST['return'];
+}
+header('Location: ' . $redirect);
+exit();