Add Finished Books feature with episode playlists

- New FinishedBook type in content.ts + finishedBooks[] field on SiteContent
- RSS parser now extracts itunes:season into each episode object
- /finished grid page and /finished/:id playlist page (filters episodes by season)
- Episodes nav link replaced with dropdown: Current Series / Finished Books
- Finished Books link added to footer
- Admin: "End Current Series & Start New Book" wizard on Current Series tab
- Admin: Finished Books management tab (add/edit/remove entries)

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

server/routes/episodes.js

@@ -29,11 +29,12 @@ function parseRssItems(xml, limit = Infinity) {
     const descText = extractCdata(descRaw).replace(/<[^>]+>/g, '').replace(/\s+/g, ' ').trim()
     const duration = (/<itunes:duration>([\s\S]*?)<\/itunes:duration>/.exec(block)?.[1] ?? '').trim()
     const episode = (/<itunes:episode>([\s\S]*?)<\/itunes:episode>/.exec(block)?.[1] ?? '').trim()
+    const season = (/<itunes:season>([\s\S]*?)<\/itunes:season>/.exec(block)?.[1] ?? '').trim()
     items.push({
       title, pubDate, link,
       audioUrl: enclosureUrl,
       description: descText.slice(0, 220) + (descText.length > 220 ? '…' : ''),
-      duration, episode,
+      duration, episode, season,
     })
   }
   return items