feat: expose per-entry disabled/loading state in Popover component for Download All button

## Summary

When a batch "Download All" is in progress in `LinearPreview.vue`, the `downloadingAll` ref guards against overlapping calls but is not reflected in the Popover entry's UI — the item still appears clickable with the default download icon during an active batch download.

## Desired Behavior

- Extend the Popover component's entry schema to accept optional `disabled` and/or `loading` props per entry.
- Wire `downloadingAll` (or a computed derived from it) into the "Download All" entry so it renders as disabled/loading while `downloadAsset` is running.

## Context

This is a **presentation-only** change — the functional concurrency guard (`downloadingAll.value` early return) already prevents overlapping batch downloads. This issue only addresses UX feedback to the user.

## Related

- PR: https://github.com/Comfy-Org/ComfyUI_frontend/pull/11410
- Review comment: https://github.com/Comfy-Org/ComfyUI_frontend/pull/11410#discussion_r3107726985

Requested by @christian-byrne.

┆Issue is synchronized with this [Notion page](https://www.notion.so/Issue-11871-feat-expose-per-entry-disabled-loading-state-in-Popover-component-for-Download-All-b-3556d73d3650819790a3c31cdc8a9a16) by [Unito](https://www.unito.io)