feat(provisioning): admin game-signup mode setting, invite link option, staff self-create

Follow-ups from live testing:

- Game-account creation is now an admin Settings control (disabled / website /
  hybrid / game) instead of a hidden on/off flag. The site offers creation for
  website+hybrid; help text notes the shard's SignupMode (Bridge.cfg) has the final
  say. game_account_signup setting widened to a 4-value enum + validated on save.
- Invites: the accept link is ALWAYS returned and shown with a Copy button, and a
  "Email the invitation" toggle lets an admin create a link-only invite (no email)
  or email it. Backend takes sendEmail (default true) and always returns acceptUrl.
- Staff can create a game account from their own /admin/characters page too
  (POST /admin/shard/account → the shared createGameAccount controller), so the
  form is reachable in both the player and admin portals.

Note: the admin Houses view (/admin/houses) already worked; the earlier failure
was a stale Vite HMR state for the new route (needs a hard refresh).

Client build clean; server routes load; swagger regenerated.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

server/src/router/v1/admin/admin.controller.js

@@ -494,6 +494,12 @@ async function updateSettings(req, res) {
   ) {
     return res.status(400).json({ message: 'Invalid player_registration value' })
   }
+  if (
+    settings.GAME_SIGNUP_KEY in updates &&
+    !settings.GAME_SIGNUP_MODES.includes(updates[settings.GAME_SIGNUP_KEY])
+  ) {
+    return res.status(400).json({ message: 'Invalid game_account_signup value' })
+  }
   // The homepage teaser is rich text (HTML) from the shared editor — sanitize it
   // against the same allowlist as post/wiki bodies so a stored value is safe (the
   // client re-sanitizes on render as defense in depth).