feat: Add pause functionality to model downloader

[imtuyethan]

Background

https://discord.com/channels/1107178041848909847/1280101987618394135

Users with slow or unreliable internet connections face challenges when downloading large AI models in Jan. Currently, the only option is to cancel the download, which can be frustrating for those who need to temporarily pause and resume downloads.

Feature request

Implement a pause button in the model downloader interface, allowing users to temporarily halt and later resume model downloads without losing progress.

• Add a "Pause" functionality next to the existing "Cancel" button in the download interface
• When paused, change the "Pause" button to a "Resume" button

Side note: @urmauur: I made some changes to the current model downloading pop-up as well, see attached image.

Figma link: https://www.figma.com/design/DYfpMhf8qiSReKvYooBgDV/Jan-App-(3rd-version)?node-id=8935-34588&t=dBXPTA4a6bGLu7FC-4

[louis-menlo]

Dup #5001