Minor bug fixes and improvements.

New: Back up and restore your data. Settings now has a "Back up your data" section - save your subscriptions, category assignments, and settings to a file, and restore them anytime (for example after a browser reset).

Fixed: On Firefox, if YouTube access wasn't turned on, importing subscriptions could silently do nothing. The dashboard now shows a clear "Enable YouTube access" button so importing always works.

Also included: videos no longer disappear from your feed on refresh - new uploads are kept as history instead of being replaced.

Fixed an issue where videos could disappear from your feed after refreshing.
- New uploads are now saved and build up as history, instead of being replaced each time.
- If YouTube temporarily rate-limits a channel, the extension retries and keeps that channel's existing videos instead of dropping them.
- Up to about 90 days of video history is now kept.