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/types/settings.ts

@@ -105,6 +105,8 @@ export type UserPreferences = {
 	autoJumpToPlayer: boolean;
 	/** Load older episodes from the Feed list: manual button or automatic at the bottom (default: manual). */
 	fetchMoreMode: FetchMoreMode;
+	/** Minutes between automatic background feed refreshes (default: 30). */
+	refreshIntervalMinutes: number;
 };
 
 export type AppState = {