Add download option and CD disc artwork to finished books; v2.0.31

- Add downloadUrl field to FinishedBook type
- Admin: paste any external URL (Drive, Dropbox, etc.) per finished book
- Public series page: replace square hero image with circular CD disc artwork
  (conic-gradient sheen, center hole, drop shadow, slight tilt)
- Stream Episodes + Download Album action buttons in the hero
- Grid cards show a gold download badge when a download URL is set

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

src/AdminPage.tsx

@@ -4190,6 +4190,10 @@ async function handleAssetUpload(event: ChangeEvent<HTMLInputElement>) {
                         <label>Season Number</label>
                         <input type="number" value={book.season} onChange={e => setForm(f => ({ ...f, finishedBooks: (f.finishedBooks ?? []).map(b => b.id === book.id ? { ...b, season: parseInt(e.target.value) || 1 } : b) }))} />
                       </div>
+                      <div className="admin-field">
+                        <label>Download URL <span style={{ fontWeight: 400, opacity: 0.7 }}>(ZIP — Google Drive, Dropbox, etc.)</span></label>
+                        <input type="url" placeholder="https://drive.google.com/…" value={book.downloadUrl ?? ''} onChange={e => setForm(f => ({ ...f, finishedBooks: (f.finishedBooks ?? []).map(b => b.id === book.id ? { ...b, downloadUrl: e.target.value } : b) }))} />
+                      </div>
                     </div>
                     <button type="button" className="btn-admin-remove" onClick={() => setForm(f => ({ ...f, finishedBooks: (f.finishedBooks ?? []).filter(b => b.id !== book.id) }))}>Remove</button>
                   </div>