Self-rescheduling refresh timer (default 30 min, configurable via a
Preferences item, re-read on every tick, skips in-flight refreshes).
fetchEpisodes returns null on network failure/timeout so a failed
refresh can never wipe a feed's episodes (addFeed/refreshFeed/
refreshAllFeeds all treat null as unchanged); feeds still refresh on
launch.
Two fixes to refresh order stability (My Shows / Feed sort by
lastUpdated):
- A refresh that fetches identical episodes no longer bumps
lastUpdated (id-set comparison via sameEpisodes), so unchanged
feeds keep their position instead of reordering every cycle.
- refreshAllFeeds now fetches in parallel and applies ONE atomic
update instead of a per-feed setFeeds, which re-sorted the list
once per completion and made order flap until the batch finished.
Adds feed-refresh regression tests with mocked clock.
