fix: conditionally show upload panels based on upload activity

[xuang7]

Purpose

This PR fixes #3804 that the upload status panel behaved unexpectedly: when there were no queued/active uploads, the UI still rendered empty panels, which was confusing. This PR hides empty panels and restores the clear empty state.

Changes

• Introduce a flag hasAnyActivity = queuedCount > 0 || activeCount > 0 || pendingChangesCount > 0
• Conditionally render status panels:
  • Pending only when queuedCount > 0
  • Uploading only when activeCount > 0
  • Finished only when hasAnyActivity
• Restore the empty state: when no activity, render <texera-dataset-staged-objects-list> outside the collapse so “No pending changes” is visible
• Add a bottom divider beneath the staged list to improve visual separation (the previous [nzBorder] was removed to avoid overlapping with the vertical divider)

Demonstration

Datasets page:

Finished panel:

Collapsed	Expanded (delete)	Expanded (adds)

Uploading files:

uploading.mov

Remove files:

remove.mov

[aglinxinyuan]

LGTM!