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/src/Database.php

@@ -1252,6 +1252,107 @@ class Database
         $this->pdo->prepare('DELETE FROM tags WHERE id = ?')->execute([$id]);
     }
 
+    // ========================================================================
+    // LANGUAGE MANAGEMENT (admin)
+    // ========================================================================
+
+    /**
+     * Search languages by name prefix. Returns up to 10 matching languages.
+     * If $query is empty, returns the most-used languages (up to 10).
+     */
+    public function searchLanguages(string $query = ''): array
+    {
+        $query = trim($query);
+        if ($query === '') {
+            $stmt = $this->pdo->query('
+                SELECT l.id, UPPER(SUBSTR(l.name,1,1)) || SUBSTR(l.name,2) as name,
+                       COUNT(DISTINCT tl.thesis_id) as thesis_count
+                FROM languages l
+                LEFT JOIN thesis_languages tl ON l.id = tl.language_id
+                GROUP BY l.id
+                ORDER BY thesis_count DESC, l.name COLLATE NOCASE
+                LIMIT 10
+            ');
+        } else {
+            $stmt = $this->pdo->prepare('
+                SELECT l.id, UPPER(SUBSTR(l.name,1,1)) || SUBSTR(l.name,2) as name,
+                       COUNT(DISTINCT tl.thesis_id) as thesis_count
+                FROM languages l
+                LEFT JOIN thesis_languages tl ON l.id = tl.language_id
+                WHERE LOWER(l.name) LIKE LOWER(?)
+                GROUP BY l.id
+                ORDER BY LOWER(l.name) = LOWER(?) DESC, thesis_count DESC, l.name COLLATE NOCASE
+                LIMIT 10
+            ');
+            $stmt->execute([$query . '%', $query]);
+        }
+        return $stmt->fetchAll();
+    }
+
+    /**
+     * Return all languages with a count of associated theses.
+     */
+    public function getAllLanguagesWithCount(): array
+    {
+        $stmt = $this->pdo->query('
+            SELECT l.id, UPPER(SUBSTR(l.name,1,1)) || SUBSTR(l.name,2) as name,
+                   COUNT(DISTINCT tl.thesis_id) as thesis_count
+            FROM languages l
+            LEFT JOIN thesis_languages tl ON l.id = tl.language_id
+            GROUP BY l.id
+            ORDER BY l.name COLLATE NOCASE
+        ');
+        return $stmt->fetchAll();
+    }
+
+    /**
+     * Rename a language. Throws if the new name already exists.
+     */
+    public function renameLanguage(int $id, string $newName): void
+    {
+        $newName = trim($newName);
+        if ($newName === '') {
+            throw new Exception('Le nom de la langue ne peut pas être vide.');
+        }
+        $stmt = $this->pdo->prepare('SELECT id FROM languages WHERE LOWER(name) = LOWER(?) AND id != ?');
+        $stmt->execute([$newName, $id]);
+        if ($stmt->fetch()) {
+            throw new Exception('Une langue avec ce nom existe déjà.');
+        }
+        $this->pdo->prepare('UPDATE languages SET name = ? WHERE id = ?')->execute([$newName, $id]);
+    }
+
+    /**
+     * Merge sourceId into targetId: reassign all thesis_languages rows, then delete source.
+     */
+    public function mergeLanguage(int $sourceId, int $targetId): void
+    {
+        if ($sourceId === $targetId) {
+            throw new Exception('Source et destination identiques.');
+        }
+        $this->pdo->beginTransaction();
+        try {
+            $this->pdo->prepare('
+                INSERT OR IGNORE INTO thesis_languages (language_id, thesis_id)
+                SELECT ?, thesis_id FROM thesis_languages WHERE language_id = ?
+            ')->execute([$targetId, $sourceId]);
+            $this->pdo->prepare('DELETE FROM thesis_languages WHERE language_id = ?')->execute([$sourceId]);
+            $this->pdo->prepare('DELETE FROM languages WHERE id = ?')->execute([$sourceId]);
+            $this->pdo->commit();
+        } catch (\Throwable $e) {
+            $this->pdo->rollBack();
+            throw $e;
+        }
+    }
+
+    /**
+     * Delete a language and all its thesis_languages rows.
+     */
+    public function deleteLanguage(int $id): void
+    {
+        $this->pdo->prepare('DELETE FROM languages WHERE id = ?')->execute([$id]);
+    }
+
     /**
      * Get orientation ID by name
      */