feat(feed): periodic background refresh with failed-fetch guard

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.

src/stores/app.ts

@@ -42,6 +42,7 @@ const defaultPreferences: UserPreferences = {
 	autoDownloadWhitelist: [],
 	autoJumpToPlayer: true,
 	fetchMoreMode: "manual",
+	refreshIntervalMinutes: 30,
 };
 
 const defaultState: AppState = {