feat: add ability to download entire season

[spacecowboy]

Adds the ability to batch download an entire season. I tried to keep the implementation minimal, reusing as much as possible where it made sense.

• For seasons, both delete and download buttons can be visible at once, because a season might be partially downloaded
• DownloadViewModel has gained the ability to download multiple items. Could also be used to batch download several movies (Movie sets?)
• On download failure, the message is displayed but delete button/download buttons are not rendered. This is existing behavior and I did not change it. If you go out and back in again, then the delete/download buttons are rendered correctly.

Here are some screenshots and such:

Partially downloaded season renders both Delete and Download buttons

Failure to download an episode in the batch will stop the batch and show message to user

Season is not downloaded, only the download button is rendered

Season is fully downloaded, only the delete button is rendered

User cancels season download

Screencast.From.2026-06-18.09-42-43.mp4

Season download runs out of space, stops and shows error to user

Screencast.From.2026-06-18.09-42-06.mp4

Fixes #73
Fixes #461
Fixes #865