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/components/GlobalActivityIndicator.tsx

@@ -0,0 +1,53 @@
+import { Show } from "solid-js";
+import { useFeedStore } from "@/stores/feed";
+import { useSearchStore } from "@/stores/search";
+import { useDownloadStore } from "@/stores/download";
+import { useActivityStore } from "@/stores/activity";
+import { LoadingIndicator } from "@/components/LoadingIndicator";
+
+/**
+ * GlobalActivityIndicator — one global top-right signal that ANY feed
+ * refresh, fetch-more, subscribe fetch, search, or download is in flight.
+ * Per-page spinners are unchanged; this overlays the content row and status
+ * bar as a single app-wide "something is happening" indicator.
+ */
+export function GlobalActivityIndicator() {
+  const feedStore = useFeedStore();
+  const searchStore = useSearchStore();
+  const downloadStore = useDownloadStore();
+  const activity = useActivityStore();
+
+  /** True while any tracked activity is in flight */
+  const isActive = () =>
+    feedStore.isLoadingFeeds() ||
+    feedStore.isLoadingMore() ||
+    searchStore.isSearching() ||
+    downloadStore.getActiveCount() + downloadStore.getQueue().length > 0 ||
+    activity.isActive();
+
+  /** Label priority: downloads in flight > latest tracked activity >
+   *  generic loading (only reachable when an isLoading/isSearching flag is
+   *  on but nothing else is). */
+  const label = () => {
+    const activeCount = downloadStore.getActiveCount();
+    const queueLength = downloadStore.getQueue().length;
+    if (activeCount + queueLength > 0) {
+      return `Downloading ${activeCount}${
+        queueLength > 0 ? ` +${queueLength} queued` : ""
+      }`;
+    }
+    if (activity.isActive()) {
+      const latest = activity.labels().at(-1);
+      return `${latest ?? "Loading"}…`;
+    }
+    return "Loading…";
+  };
+
+  return (
+    <Show when={isActive()}>
+      <box position="absolute" top={0} right={0} paddingRight={1}>
+        <LoadingIndicator label={label()} />
+      </box>
+    </Show>
+  );
+}