[Feature Request / Enhancement] Download queue / task list to browse other mods while downloading

### Problem

In the current download interface, when you start downloading a mod, the UI stays on that page and you cannot browse or view other mods until the download finishes. This is inconvenient for large mods or slow connections—users are forced to wait idly.

### Desired behavior

Add a **download task list** (queue) with the following capabilities:

1. Clicking "Download" adds the mod to a background queue without blocking the UI.
2. Users can freely switch back to the mod list to browse, view details, or even start other downloads.
3. Download progress is shown in a floating panel, sidebar, or status indicator (e.g., number of pending tasks, progress bars).
4. (Optional) Support pause/cancel for individual tasks.

### Use cases

- Browsing screenshots and descriptions of other mods while waiting for a large map pack.
- Queuing multiple mods without waiting for each one to finish sequentially.

Start by tracing the current download interface and download lifecycle in the project. Define how downloads move into a background queue while the user returns to the mod list, and how progress is exposed in the interface. Done means users can browse and queue multiple mods without waiting, with visible task progress; pause and cancel are optional.

Written by the indexing model from the issue text.