Add function to pack the already downloaded chapter to CBZ

[just-shadyumbrella]

Describe your suggested feature

May not really a good idea, I'm just don't want to redownload the entire chapters. Or should I just convert it to CBZ manually? I thought the chapters were registered internally in app so it quite impossible to make it in current implementation.

Other details

No response

Acknowledgements

• [x] I have searched the existing issues and this is a new ticket, NOT a duplicate or related to another open or closed issue.
• [x] I have written a short but informative title.
• [x] I have updated the app to version 0.18.0.
• [x] I will fill out all of the requested information in this form.