Harden file browser: cap preview lines, fix race conditions, download UX

- Cap text preview at 5,000 lines with truncation footer and download link
  to prevent browser freeze on large files (300k+ DOM nodes)
- Add request generation counters to discard stale API responses from
  rapid directory/file clicking
- Guard initial $effect with hasInitiallyLoaded to prevent double-load
- Add download loading state with spinner and disabled button
- Delay URL.revokeObjectURL by 5s so browser can start download

frontend/src/lib/api/files.ts

@@ -120,5 +120,6 @@ export async function downloadFile(sandboxId: string, path: string, filename: st
 	document.body.appendChild(a);
 	a.click();
 	a.remove();
-	URL.revokeObjectURL(url);
+	// Delay revocation so the browser has time to start the download
+	setTimeout(() => URL.revokeObjectURL(url), 5000);
 }