Support cancel/delete for pending uploads

[xuang7]

Problem Statement

Currently, we organize upload tasks into three panels: Pending, Uploading, and Finished. Only the tasks atUploading panel can be removed. The queued uploads in the Pending panel cannot be canceled or deleted before they start uploading, which makes it inconvenient when users want to clear or reprioritize queued files.

Proposed Solution

1. Allow users to remove or cancel uploads directly from the Pending panel before they start uploading. Ensure removing a pending upload updates both the UI and internal task queue state consistently.