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/assets/css/admin.css

@@ -74,6 +74,16 @@
     padding: 0 0 var(--space-2xl);
 }
 
+#admin-table-wrap table thead {
+    position: sticky;
+    top: 0;
+    z-index: 5;
+}
+
+#admin-table-wrap table thead th {
+    background: var(--bg-primary);
+}
+
 .admin-body main > table tbody tr:nth-child(even) {
     background: var(--bg-secondary);
 }