Consolidate admin tools into an "Admin" mode with its own sub-navbar

Why

The main header nav had grown 5+ admin links (Transactions, Audit, API keys, Groups, Webhooks — and /admin/backfill wasn't even surfaced). Collapse them into a single Admin entry that opens a dedicated admin section with its own sub-navbar.

Change (economy-explorer)

• components/HeaderNav.tsx: replace the 5 admin links with one Admin → /admin (highlights for /admin/* and /transactions).
• components/AdminNav.tsx: the admin sub-navbar (Overview, Transactions, Audit, API keys, Groups, Webhooks, Backfill) with active-highlighting.
• app/admin/layout.tsx: gates the whole /admin/* area to admins (non-admins never see the tool list) and renders AdminNav above each page.
• app/admin/page.tsx: an admin landing/overview with a card per tool.
• /transactions (admin-gated but lives outside /admin, since its [txnId] detail isn't admin-only) renders AdminNav itself so the admin section feels continuous.
• CSS: .admin-subnav + .admin-tool-grid in explorer.css.

Individual admin pages keep their own gate as defence-in-depth. Follow-up to PAR-160 (which added /admin/webhooks).