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>

backend/Cargo.lock

@@ -1558,7 +1558,7 @@ checksum = "c41e0c4fef86961ac6d6f8a82609f55f31b05e4fce149ac5710e439df7619ba4"
 
 [[package]]
 name = "mangalord"
-version = "0.125.0"
+version = "0.125.1"
 dependencies = [
  "anyhow",
  "argon2",

backend/Cargo.toml

@@ -1,6 +1,6 @@
 [package]
 name = "mangalord"
-version = "0.125.0"
+version = "0.125.1"
 edition = "2021"
 default-run = "mangalord"