feat(admin): manage timeslot link + per-order download on orders-by-timeslot

[choden-dev]

Summary

Two small admin improvements to the Orders by Pickup Timeslot view (components/admin/OrdersByTimeslotView.tsx):

1. Open the timeslot document — each timeslot header now has a ⚙ Manage Timeslot link that opens /admin/collections/timeslots/{id}, the Payload document edit view. This is where the existing DeleteTimeslotButton (delete + notify customers) lives, so admins can jump straight there from the list.
2. Download all files for a single order — added a ⬇ Download All button in each order's Actions column. It reuses the shared download logic (parallel presign, sequential open with a small delay to avoid popup blockers). Only shown when the order has downloadable files.

Implementation notes

• Refactored the existing per-timeslot download into a shared downloadFilesForOrders(orders) helper. Both handleDownloadAllFiles (whole timeslot) and the new handleDownloadOrderFiles (single order) delegate to it, keeping behaviour identical.
• The in-place delete was intentionally not implemented in this view: DeleteTimeslotButton depends on Payload's useDocumentInfo() hook, which is only available inside a Payload document context. Linking to the document view is the correct integration point.

Testing

• tsc --noEmit passes with no errors.
• biome check clean.