fix: bookmark toggle is optimistic and no longer fails silently

The detail-page bookmark toggle awaited the round-trip before updating and
had no catch — a failed create/delete threw unhandled and the user saw
nothing. It now flips optimistically (placeholder reconciled with the server
row on success), rolls back on error, and surfaces a toast, matching the
sibling like/dislike buttons.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

frontend/package.json

@@ -1,6 +1,6 @@
 {
   "name": "mangalord-frontend",
-  "version": "0.115.0",
+  "version": "0.115.1",
   "private": true,
   "type": "module",
   "scripts": {