fix: make bookmark add idempotent instead of 409

Re-adding an already-bookmarked manga returned 409, which the UI surfaced as
a false "Could not add bookmark" toast — collections are idempotent, bookmarks
weren't. On a unique violation, fetch and return the existing bookmark, and
answer 200 (vs 201 for a fresh insert). The frontend toggle already treats a
successful POST as done, so the false toast disappears with no client change.

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

frontend/package.json

@@ -1,6 +1,6 @@
 {
   "name": "mangalord-frontend",
-  "version": "0.124.20",
+  "version": "0.124.21",
   "private": true,
   "type": "module",
   "scripts": {