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/pages/MyShows/MyShowsPage.tsx

@@ -346,7 +346,7 @@ export function MyShowsPage() {
 					</For>
 					<Show when={feedStore.isLoadingMore()}>
 						<box paddingLeft={2} paddingTop={1}>
-							<LoadingIndicator />
+							<LoadingIndicator label="Loading more…" />
 						</box>
 					</Show>
 				</Show>