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/api/rss-parser.ts

@@ -82,6 +82,16 @@ export const getRSSItems = (xml: string): string[] => {
   return channel.match(/<item[\s\S]*?<\/item>/gi) ?? []
 }
 
+/** Channel-level artwork: `<itunes:image href>` (podcasts) or RSS 2.0
+ *  `<image><url>`. Exported so the feed store can backfill a feed's
+ *  coverUrl on refresh without re-deriving the channel block. */
+export const parseChannelCoverUrl = (channel: string): string | undefined => {
+  const itunesHref = getAttr(channel, "itunes:image", "href")
+  if (itunesHref) return itunesHref
+  const url = getTagValue(channel, "image").match(/<url>([\s\S]*?)<\/url>/i)?.[1]
+  return url?.trim() || undefined
+}
+
 /** Parse a single `<item>` into an Episode. Exported so the feed store can
  *  parse large feeds in bounded chunks (yielding to the event loop between
  *  chunks) instead of one synchronous block. */
@@ -158,6 +168,7 @@ export const parseRSSFeed = (xml: string, feedUrl: string): Podcast & { episodes
     feedUrl,
     lastUpdated,
     isSubscribed: true,
+    coverUrl: parseChannelCoverUrl(channel),
     episodes,
   }
 }