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/stores/activity.ts

@@ -0,0 +1,75 @@
+/**
+ * Activity store for PodTUI
+ *
+ * Shared leak-proof activity counter: any store can surface "something is
+ * loading/downloading" to the global top-right indicator. beginActivity
+ * returns an end token that removes exactly THAT instance, so concurrent
+ * overlapping activities compose correctly; prefer track() so callers
+ * cannot strand the counter.
+ */
+
+import { createSignal } from "solid-js";
+
+/** Create activity store */
+function createActivityStore() {
+	const [count, setCount] = createSignal(0);
+	const [labels, setLabels] = createSignal<string[]>([]);
+
+	/** Begin a tracked activity and return its end function. Every begin
+	 *  MUST be paired with exactly one call of the returned end (via the
+	 *  token); prefer track() so the pairing is automatic. Duplicate labels
+	 *  are allowed — each end removes exactly one instance (found by
+	 *  indexOf). */
+	const beginActivity = (label: string): (() => void) => {
+		setLabels((prev) => [...prev, label]);
+		setCount((c) => c + 1);
+		let ended = false;
+		return () => {
+			if (ended) return;
+			ended = true;
+			setLabels((prev) => {
+				const idx = prev.indexOf(label);
+				if (idx === -1) return prev;
+				const next = [...prev];
+				next.splice(idx, 1);
+				return next;
+			});
+			setCount((c) => Math.max(0, c - 1));
+		};
+	};
+
+	/** Track a promise: begin an activity, auto-end when it settles, and
+	 *  re-throw on rejection so the caller's error handling is untouched. */
+	const track = async <T,>(p: Promise<T>, label: string): Promise<T> => {
+		const end = beginActivity(label);
+		try {
+			return await p;
+		} finally {
+			end();
+		}
+	};
+
+	/** True while at least one activity is in flight */
+	const isActive = (): boolean => count() > 0;
+
+	return {
+		// State
+		count,
+		labels,
+		// Actions
+		beginActivity,
+		track,
+		// Getters
+		isActive,
+	};
+}
+
+/** Singleton activity store */
+let activityStoreInstance: ReturnType<typeof createActivityStore> | null = null;
+
+export function useActivityStore() {
+	if (!activityStoreInstance) {
+		activityStoreInstance = createActivityStore();
+	}
+	return activityStoreInstance;
+}