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>

backend/Cargo.lock

@@ -1558,7 +1558,7 @@ checksum = "c41e0c4fef86961ac6d6f8a82609f55f31b05e4fce149ac5710e439df7619ba4"
 
 [[package]]
 name = "mangalord"
-version = "0.115.0"
+version = "0.115.1"
 dependencies = [
  "anyhow",
  "argon2",

backend/Cargo.toml

@@ -1,6 +1,6 @@
 [package]
 name = "mangalord"
-version = "0.115.0"
+version = "0.115.1"
 edition = "2021"
 default-run = "mangalord"