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/templates/admin/index-table.php

@@ -31,6 +31,8 @@ $sortArrow = function(string $col) use ($sortCol, $sortDir): string {
     <div class="admin-bulk-btns">
         <button type="button" class="btn btn--sm btn--green admin-btn-publish" onclick="confirmBulk('publish')">Publier</button>
         <button type="button" class="btn btn--sm btn--muted admin-btn-unpublish" onclick="confirmBulk('unpublish')">Dépublier</button>
+        <button type="button" class="btn btn--sm" onclick="confirmExport()">Exporter CSV</button>
+        <button type="button" class="btn btn--sm" onclick="confirmExportFiles()">Exporter fichiers</button>
         <button type="button" class="btn btn--sm btn--red admin-btn-delete" onclick="confirmBulk('delete')">Supprimer</button>
     </div>
 </div>