feat: enable canceling pending uploads before upload begins

[xuang7]

Purpose

This PR resolved #3844 that pending uploads cannot be removed before they start. This PR enables removing/canceling items directly from the Pending panel, improving queue control and flexibility in managing uploads.

Changes

• Add a Remove action to Pending items; behavior and styling match the Uploading panel’s remove action.
• Refactor cancelExistingUpload(fileName: string):
  • Uploading/Initializing → reuse the abort path to properly finalize server-side and prevent leaks.
  • Pending → front-end clean only (remove from queue, tasks) with no backend abort call.

Demonstration

remove.mov

[aglinxinyuan]

LGTM!