Setting filter expiry to 'Never' doesn't stick

[d-meagher]

Describe the bug

When setting the expiry time for a filter choosing Never doesn't persist. All other date/time based expiry settings work as intended.

To Reproduce

1. Open a filter
2. Set the duration for expiry to Never
3. Go back to the list of filters or quit the app
4. Reopen the filter and check the expiry duration

Expected behaviour

The filter expiry duration will save as Never

Screenshots

setting.filter.expiry.mp4

Instance address

mastodon.social

Device

iPhone 14 Pro

OS

iOS 18

Version

Public Beta 5

Additional context

Mammoth version 2.15
Build 502