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>

package.json

@@ -1,7 +1,7 @@
 {
   "name": "siteforge",
   "private": true,
-  "version": "2.0.30",
+  "version": "2.0.31",
   "type": "module",
   "scripts": {
     "dev": "vite",