Feature request: GUI download should keep the source filename (file.md / converted_docs.zip are generic)

**Environment:** docling-serve v1.32.0 (Docker image `quay.io/docling-project/docling-serve:v1.32.0`, self-hosted, UI enabled, Gradio 6.26.0)

## Problem

When converting a file through the built-in Gradio UI, the downloaded result always gets a generic name instead of the uploaded source filename. With several documents converted one after another this makes the downloads hard to tell apart.

## Repro

1. Open the UI (`/ui`), go to the "Convert File" tab.
2. Upload a single file named e.g. `invoice_2026.pdf`.
3. Click "Process File".

**Without "Return as File" checked:**
- Markdown output appears in the in-page code box.
- Downloading it saves a file named `file.md` (generic), not `invoice_2026.md`.

**With "Return as File" checked:**
- Download saves `converted_docs.zip`.
- Inside the zip the markdown *is* correctly named `invoice_2026.md` – only the archive name is generic.

## Expected

The downloaded artifact should carry the original source filename, e.g. `invoice_2026.md` for a single-file conversion (or `invoice_2026.zip` when returned as archive). That way parallel conversions of different documents remain distinguishable.

## Notes

- The inner file of the archive already keeps the source stem – only the outer names (`file.md`, `converted_docs.zip`) are hardcoded/generic.
- This matters most for single-file conversions; with multiple uploads some generic fallback is of course fine.

*Posted by Hermes Agent in collaboration with zunami — knowledge gathered through real-world production operation.*

Start in the Gradio UI implementation for the Convert File tab and trace how the uploaded filename is passed to the Markdown and archive download outputs. Verify the single-file cases produce the source stem with the appropriate extension, while multiple uploads retain a generic fallback, then test both Return as File settings.

Written by the indexing model from the issue text.