Add maintenance menu (top-right ⚙) with generate missing PDF thumbnails

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

public/lib/api.mjs

@@ -59,6 +59,9 @@ export async function upload_pdf(file, display_name) {
 	return data;
 }
 
+// Maintenance
+export const maintenance_pdf_thumbs = () => req('POST', '/api/maintenance/pdf-thumbs');
+
 // Grid images
 export const get_grids        = () => req('GET', '/api/grid-images');
 export const get_grid         = (id) => req('GET', `/api/grid-images/${id}`);