feat(feed): bound feed lifecycle to 30-day window with nonblocking refresh

Persisted feeds keep only episodes from the last 30 days (plus completed
downloads); older episodes live in volatile memory and survive refreshes
via union merge, with per-feed in-memory caches capped at 500. Refresh
batches run at FETCH_CONCURRENCY=4 with per-feed incremental apply (no
Promise.all barrier), config.json writes are trailing-edge debounced
(250ms, immediate flushPendingSave for unsubscribes), and cold
fetch-more refetches abort at FETCH_TIMEOUT_MS. A shared activity store
powers a global top-right indicator covering refresh, fetch-more,
subscribe, search, and downloads.

Also includes the in-flight incremental RSS parsing (chunked with
event-loop yields) and refresh spinner work this tree already carried.

src/pages/Feed/FeedPage.tsx

@@ -236,7 +236,7 @@ function FeedPage() {
 		<Show
 			when={episodes().length > 0}
 			fallback={
-				<box padding={1}>
+				<box padding={1} alignItems="center">
 					<Show
 						when={feedStore.isLoadingFeeds()}
 						fallback={
@@ -355,7 +355,7 @@ function FeedPage() {
 				</box>
 			</Show>
 			<Show when={feedStore.isLoadingFeeds()}>
-				<box paddingLeft={2} paddingTop={1}>
+				<box alignItems="center" paddingTop={1}>
 					<LoadingIndicator label="Refreshing…" />
 				</box>
 			</Show>

src/pages/Player/PlayerPage.tsx

@@ -133,6 +133,7 @@ export function PlayerPage() {
 			currentLabel="Player"
 			panes={2}
 			focused={isActive}
+			currentBorder={["left"]}
 		/>
 	);
 }