feat(admin-console): Stage 6 — mail & notifications domain

Add the mail, notifications, and broadcast pages over the mail, notification,
and diplomail services (no new business logic), completing the operator console.

- GET  /_gm/mail                         deliveries (paginated) + dead-letters
- GET  /_gm/mail/deliveries/{id}         delivery detail + attempts
- POST /_gm/mail/deliveries/{id}/resend  re-enqueue a non-sent delivery
- GET  /_gm/notifications                notifications + dead-letters + malformed
- GET/POST /_gm/broadcast                multi-game admin diplomatic broadcast

Console depends on MailAdmin / NotificationAdmin / DiplomailAdmin interfaces
(satisfied by the concrete services); pages render in tests without a database.
Delivery detail and dead-letters live under /_gm/mail/deliveries/* and
/_gm/mail/... static segments to avoid a param/static route conflict. Resend
and broadcast flow through the CSRF guard.

Tests: mail page, delivery detail (+ not-found), resend (+ bad-CSRF),
notifications overview, broadcast form + send (input assertions) + bad game
ids, and unavailable. Plus an integration test that drives /_gm end to end
through the real gateway → backend (401 challenge + authenticated dashboard).

Docs: backend/docs/admin-console.md page inventory completed.

backend/internal/adminconsole/assets/console.css

@@ -100,3 +100,4 @@ button.danger { background: var(--danger); color: #1a0606; }
 code { background: var(--bg); padding: 0.05rem 0.3rem; border-radius: 4px; }
 .actions { display: flex; flex-wrap: wrap; gap: 0.6rem; margin: 0.8rem 0; }
 .actions form { margin: 0; }
+.subnav { color: var(--ink-dim); margin: -0.3rem 0 1rem; font-size: 0.9rem; }