Add a context menu in sticker picker that contains a remove action, which will prompt the user whether they want to remove the sticker.

Type: add

Adapted from the output of OpenCode (GLM 5.2). tusooa reviewed the code and takes full responsibility.

Verify you can right-click (desktop) / long press (mobile) on the sticker to invoke the context menu. Click on the "Remove sticker..." action. Verify that a confirmation dialog pops up. Click "Remove" and verify the sticker is removed if you have the permission to do so.