fix: confirm admin role toggle and keep the checkbox controlled

The admin checkbox fired an immediate, irreversible privilege change with no
confirmation, and on cancel/failure the optimistic DOM flip stuck while the
server state was unchanged. Add a confirm() gate (like Delete) and make the
checkbox controlled by u.is_admin: onchange reverts the DOM immediately and it
only flips once the server confirms — so a cancelled or failed toggle never
misrepresents the real role.

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

frontend/package.json

@@ -1,6 +1,6 @@
 {
   "name": "mangalord-frontend",
-  "version": "0.125.0",
+  "version": "0.125.1",
   "private": true,
   "type": "module",
   "scripts": {