feat(nav): give admins a mobile entry point to the admin area

The admin area was reachable only from the desktop header's is_admin link;
on mobile there was no tab or row, so admins had to type the URL and no tab
highlighted. Add a conditional Admin row to the mobile Account hub and
`/admin` to the Account tab's match prefixes, so the tab stays highlighted
while an admin is in the admin area.

e2e: an admin session shows an Admin row linking to /admin; a non-admin
never sees it.

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

backend/Cargo.lock

@@ -1517,7 +1517,7 @@ checksum = "c41e0c4fef86961ac6d6f8a82609f55f31b05e4fce149ac5710e439df7619ba4"
 
 [[package]]
 name = "mangalord"
-version = "0.88.0"
+version = "0.89.0"
 dependencies = [
  "anyhow",
  "argon2",

backend/Cargo.toml

@@ -1,6 +1,6 @@
 [package]
 name = "mangalord"
-version = "0.88.0"
+version = "0.89.0"
 edition = "2021"
 default-run = "mangalord"