fix(memory): bound visualizer PCM cache and feed episode cache

The visualizer's PCM cache decoded the entire episode into RAM (22050 Hz
mono s16 ~160 MB/hr of audio) and held it until stop() — a 3-hour episode
pinned ~500 MB and long-form content hit 2.5 GB. The 4x decode also pulled
the whole remote file even when only minutes were listened to.

- audio-pcm-cache: sliding window around the playback position — the
  decode head caps at maxAheadSec (600s) ahead of the cursor, segments
  older than keepBehindSec (300s) are pruned, and the tail refills as
  playback advances. Steady state ~40 MB regardless of episode length;
  a backward seek past the window restarts a segment there (the existing
  seek-hole mechanism, no new failure mode).
- feed: cap the full-parse episode cache at 1000 episodes/feed so
  archive-heavy subscriptions can't pin their entire history in RAM;
  the visible list stays bounded by the user's cache preference and
  fetch-more keeps working within the ceiling.
- tests: pin the new head-cap and prune contracts (8/8 in
  audio-pcm-cache.test.ts; full suite 193 pass).

Also includes the in-flight cleanup/refactor pass (cover-art resolve
helper, page and comment tightening, ESLint config removal).

src/pages/Settings/SettingsPage.tsx

@@ -140,7 +140,6 @@ export function SettingsPage() {
 	function open() {
 		const d = depth();
 		if (d === 0) {
-			// drill into the focused section's items
 			const id = focusedSection().id;
 			nav.pushDepth({
 				kind: `settings:${id}`,
@@ -206,7 +205,6 @@ export function SettingsPage() {
 	function step(delta: number) {
 		const d = depth();
 		if (d === 2) {
-			// editor: j/k nudges the value
 			const it = editorItem();
 			if (it?.kind === "number" || it?.kind === "select")
 				it.cycle?.(delta as -1 | 1);
@@ -220,7 +218,6 @@ export function SettingsPage() {
 		pane: PaneId;
 		mode: NavMode;
 	}) => {
-		// ignore actions meant for non-center panes
 		if (data.pane !== DEPTH_CENTER_PANE) return;
 		if (nav.activePane() !== DEPTH_CENTER_PANE) return;
 		const handler = PAGE_ACTIONS[data.action];