fix(cover+downloads): channel art parsing, episode image fallback, local playback

The Fifth Column (and any feed added by URL) had NO coverUrl — the RSS
parser never captured channel artwork, so Now Playing had nothing to show.
- rss-parser: parseChannelCoverUrl (<itunes:image href> / RSS2 <image><url>);
  parseRSSFeed sets it on the Podcast.
- feed store: fetchEpisodes returns the channel cover; subscribe + both
  refresh paths backfill coverUrl when missing (no second fetch).
- useAudio + CLI --play: cover resolves feed.podcast.coverUrl ?? episode.imageUrl,
  so episodes without channel art still get their own image.
- useAudio play/load/switchBackend: prefer the downloaded file
  (getDownloadedFilePath) over the stream URL — downloaded episodes now
  play from disk.
Verified: Fifth Column episode, cold cache -> cover-art-files set at load
-> albumart track present.

src/index.tsx

@@ -186,9 +186,13 @@ async function handlePlay(feeds: Feed[], arg: string): Promise<void> {
 		const backend = createAudioBackend()
 		if (episodeResult.audioUrl) {
 			// Stage the podcast cover so the system Now Playing shows
-			// artwork (mpv --cover-art-files), like the UI path does.
-			const coverArtPath = feedResult.podcast.coverUrl
-				? await fetchCoverArt(feedResult.podcast.coverUrl)
+			// artwork (mpv --cover-art-files), like the UI path does. Falls
+			// back to the episode's own image when the feed has no channel
+			// cover (URL-added feeds).
+			const coverUrl =
+				feedResult.podcast.coverUrl ?? episodeResult.imageUrl;
+			const coverArtPath = coverUrl
+				? await fetchCoverArt(coverUrl)
 				: null
 			await backend.play(episodeResult.audioUrl, {
 				mediaTitle: `${feedResult.podcast.title} — ${episodeResult.title}`,