feat: labeled loading spinners + [Fetch More] pagination on the feed list

Loading indicators: the braille spinner now carries a contextual label
(Refreshing…, Fetching…, Loading more…, Discovering…, Searching…) and is
shown in every loading state that previously rendered nothing — Discover
results, Search results fallback, and the empty Feed list.

Feed pagination: a focusable "[Fetch More]" row at the bottom of the flat
feed list advances every feed's loaded window by 50 episodes via the new
loadMoreAllFeeds/hasMoreAcrossAll store API. Behavior is a setting
(Fetch More: manual|auto, default manual) persisted in config.json; auto
fetches when focus reaches the bottom row. The button row is excluded
from episode focus so no episode is double-highlighted while it is active.

src/types/settings.ts

@@ -84,11 +84,16 @@ export type AppSettings = {
 	visualizer: VisualizerSettings;
 };
 
+/** How the Feed list loads older episodes (default: manual "[Fetch More]"). */
+export type FetchMoreMode = "manual" | "auto";
+
 export type UserPreferences = {
 	showExplicit: boolean;
 	autoDownload: boolean;
 	/** Jump to the Player view automatically when playback starts (default: true) */
 	autoJumpToPlayer: boolean;
+	/** Load older episodes from the Feed list: manual button or automatic at the bottom (default: manual). */
+	fetchMoreMode: FetchMoreMode;
 };
 
 export type AppState = {