Add Finished Books feature with episode playlists and nav dropdown

Public:
- /finished page with ornament heading, intro text, book grid, and styled empty state
- /finished/:id episode playlist page filtered by RSS season number
- Episodes nav becomes click-toggle dropdown (Current Series / Finished Books)
- Finished Books link in footer
- Dropdown font fixed to match other nav links

Admin:
- End Current Series & Start New Book wizard on Current Series tab
- Finished Books management tab with add/edit/remove and image asset picker

Data:
- FinishedBook type + finishedBooks[] field on SiteContent
- RSS parser extracts itunes:season per episode

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

src/AdminPage.tsx

@@ -3831,6 +3831,7 @@ export default function AdminPage({ content, onSave, onLogout }: Props) {
                       <div className="admin-field">
                         <label>Cover Image URL</label>
                         <input type="text" value={book.imageUrl} onChange={e => setForm(f => ({ ...f, finishedBooks: (f.finishedBooks ?? []).map(b => b.id === book.id ? { ...b, imageUrl: e.target.value } : b) }))} />
+                        {renderImageAssetSelector(book.imageUrl, value => setForm(f => ({ ...f, finishedBooks: (f.finishedBooks ?? []).map(b => b.id === book.id ? { ...b, imageUrl: value } : b) })), `finished-book-img-${book.id}`)}
                         {renderImagePreview(book.imageUrl, `${book.title} cover`)}
                       </div>
                       <div className="admin-field">