feat(admin): add Network Health panel to observability dashboard #698

Summary

Add a Network Health section to /admin dashboard showing external probe results alongside existing live KPI metrics.
Part of #667.

Problem

Phase 2 dashboard (#666) shows internal metrics (EPS, connections, CPU/RAM) but not external reachability (DNS, TLS expiry, public WebSocket latency).

Goal

• New Network Health panel/tab in resources/admin/dashboard.html
• Fetch from GET /admin/network-health on load and poll refresh (align interval with /admin/metrics SSE or separate polling)
• Display:
  • WebSocket RTT (open; show read/write as "—" until v2)
  • DNS records + TTL
  • SSL status + expiry countdown (warn when < 14 days)
  • Last probe time + overall success/failure badge
  • Network type badge (clearnet / tor / i2p)
  • NIP-11 name + supported NIPs mismatch warning if probe detects drift
• Manual smoke test on regtest/local with admin enabled