feat: add unauthenticated web dashboard showing monitoring state and recent alerts

Add a read-only web dashboard at GET / that displays:
- Summary counts for all monitored resources
- Domain nameserver state with per-NS records and status
- Hostname DNS records per authoritative nameserver
- TCP port open/closed state with associated hostnames
- TLS certificate details (CN, issuer, expiry, status)
- Last 100 alerts in reverse chronological order

Every data point shows relative age (e.g. '5m ago') for freshness.
Page auto-refreshes every 30 seconds via meta refresh.

Uses Tailwind CSS via CDN for a dark, technical aesthetic with
saturated teals and blues on dark slate. Single page, no navigation.

Implementation:
- internal/notify/history.go: thread-safe ring buffer (last 100 alerts)
- internal/notify/notify.go: record alerts in history before dispatch,
  refactor SendNotification into smaller dispatch helpers (funlen)
- internal/handlers/dashboard.go: template rendering with embedded HTML,
  helper functions for relative time, record formatting, expiry days
- internal/handlers/templates/dashboard.html: Tailwind-styled dashboard
- internal/handlers/handlers.go: add State and Notify dependencies
- internal/server/routes.go: register GET / dashboard route
- README.md: document dashboard and new / endpoint

No secrets (webhook URLs, API tokens, notification endpoints) are
exposed in the dashboard.

closes #82

internal/server/routes.go

@@ -22,6 +22,9 @@ func (s *Server) SetupRoutes() {
 	s.router.Use(s.mw.CORS())
 	s.router.Use(chimw.Timeout(requestTimeout))
 
+	// Dashboard (read-only web UI)
+	s.router.Get("/", s.handlers.HandleDashboard())
+
 	// Health check
 	s.router.Get("/health", s.handlers.HandleHealthCheck())