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/handlers/dashboard_test.go

@@ -0,0 +1,80 @@
+package handlers_test
+
+import (
+	"testing"
+	"time"
+
+	"sneak.berlin/go/dnswatcher/internal/handlers"
+)
+
+func TestRelTime(t *testing.T) {
+	t.Parallel()
+
+	tests := []struct {
+		name string
+		dur  time.Duration
+		want string
+	}{
+		{"zero", 0, "never"},
+		{"seconds", 30 * time.Second, "30s ago"},
+		{"minutes", 5 * time.Minute, "5m ago"},
+		{"hours", 2*time.Hour + 15*time.Minute, "2h 15m ago"},
+		{"days", 48*time.Hour + 3*time.Hour, "2d 3h ago"},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			t.Parallel()
+
+			var input time.Time
+			if tt.dur > 0 {
+				input = time.Now().Add(-tt.dur)
+			}
+
+			got := handlers.RelTime(input)
+			if got != tt.want {
+				t.Errorf(
+					"RelTime(%v) = %q, want %q",
+					tt.dur, got, tt.want,
+				)
+			}
+		})
+	}
+}
+
+func TestExpiryDays(t *testing.T) {
+	t.Parallel()
+
+	// 10 days from now.
+	future := time.Now().Add(10 * 24 * time.Hour)
+
+	days := handlers.ExpiryDays(future)
+	if days < 9 || days > 10 {
+		t.Errorf("expected ~10 days, got %d", days)
+	}
+
+	// Already expired.
+	past := time.Now().Add(-24 * time.Hour)
+
+	days = handlers.ExpiryDays(past)
+	if days != 0 {
+		t.Errorf("expected 0 for expired, got %d", days)
+	}
+}
+
+func TestFormatRecords(t *testing.T) {
+	t.Parallel()
+
+	got := handlers.FormatRecords(nil)
+	if got != "-" {
+		t.Errorf("expected -, got %q", got)
+	}
+
+	got = handlers.FormatRecords(map[string][]string{
+		"A": {"1.2.3.4"},
+	})
+
+	if got != "A: 1.2.3.4" {
+		t.Errorf("unexpected format: %q", got)
+	}
+}