Add server-side image thumbnails (sharp): /uploads/thumb/<name> route, ?thumb=1 on file endpoints, disk-cached 480px WebP previews; grids now load thumbnails with lazy loading

public/js/files.js

@@ -34,7 +34,7 @@ function humanSize(b) {
 function fileCardHTML(f) {
   const url = `${API}/api/files/${f.token}`;
   const thumb = isImageFile(f.name)
-    ? `<img src="${url}" data-img="${esc(url)}" alt="">`
+    ? `<img src="${thumbSrc(url)}" data-img="${esc(url)}" loading="lazy" decoding="async" alt="">`
     : `<div class="file-icon">📄</div>`;
   const target = isImageFile(f.name)
     ? `href="#" data-img="${esc(url)}"`