Index page: remove Mots-clés button, move export to bulk selection, fix ZipArchive error, move DB export to paramètres, sticky thead

- Remove 'Mots-clés' button from toolbar (redundant with admin sidebar tags)
- Replace export dialog with 'Exporter CSV' + 'Exporter fichiers' buttons in bulk selection bar
- Export dispatcher now accepts ?ids=1,2,3 for per-selection export
- All ExportController/Database methods accept optional thesisIds array
- Graceful error message when ZipArchive extension is missing on server
- Move DB export (SQLite download) to paramètres → Maintenance section
- Sticky table column headers (position: sticky, top: 0, z-index: 5) for index page table

app/public/admin/actions/export.php

@@ -21,6 +21,18 @@ $doCsv   = !empty($_GET['csv']);
 $doFiles = !empty($_GET['files']);
 $doDb    = !empty($_GET['db']);
 
+// Optional: filter by selected thesis IDs (bulk selection export)
+$idsRaw = $_GET['ids'] ?? '';
+$selectedIds = [];
+if ($idsRaw !== '') {
+    foreach (explode(',', $idsRaw) as $id) {
+        $id = (int) trim($id);
+        if ($id > 0) {
+            $selectedIds[] = $id;
+        }
+    }
+}
+
 if (!$doCsv && !$doFiles && !$doDb) {
     $doCsv = true;
 }
@@ -40,7 +52,7 @@ if ($doCsv) {
 
     $out = fopen('php://output', 'w');
     fputcsv($out, ExportController::CSV_HEADERS, ',', '"', '');
-    $rows = $controller->exportAllTheses();
+    $rows = $controller->exportAllTheses($selectedIds);
     foreach ($rows as $csvLine) {
         fputcsv($out, $csvLine, ',', '"', '');
     }
@@ -53,10 +65,14 @@ if ($doCsv) {
 }
 
 if ($doFiles) {
+    if (!class_exists('ZipArchive')) {
+        http_response_code(500);
+        exit('Module PHP <code>zip</code> non installé sur le serveur. Contactez l\'administrateur système (<code>apt install php8.4-zip</code>).');
+    }
     try {
-        $zipPath = $controller->createExportZip();
+        $zipPath = $controller->createExportZip($selectedIds);
         $fileSize = filesize($zipPath);
-        $fileCount = count($controller->getAllThesisFiles());
+        $fileCount = count($controller->getAllThesisFiles($selectedIds));
 
         AdminLogger::make()->logFilesExport($fileCount, (int)$fileSize);