feat(web): add Stirling-PDF-style file preview, review panel, and tool chaining UX

After upload: dropzone replaced by image viewer with zoom controls.
After processing: review panel shows result preview, file info, download,
undo button, and suggested next tools for chaining.

apps/web/src/lib/download.ts

@@ -0,0 +1,15 @@
+export function triggerDownload(url: string, filename: string) {
+  const a = document.createElement("a");
+  a.href = url;
+  a.download = filename;
+  document.body.appendChild(a);
+  a.click();
+  document.body.removeChild(a);
+}
+
+export function formatFileSize(bytes: number): string {
+  if (bytes < 1024 * 1024) {
+    return `${(bytes / 1024).toFixed(0)} KB`;
+  }
+  return `${(bytes / (1024 * 1024)).toFixed(1)} MB`;
+}