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

README.md

@@ -124,12 +124,34 @@ includes:
 - State is written atomically (write to temp file, then rename) to prevent
   corruption.
 
+### Web Dashboard
+
+dnswatcher includes an unauthenticated, read-only web dashboard at the
+root URL (`/`). It displays:
+
+- **Summary counts** for monitored domains, hostnames, ports, and
+  certificates.
+- **Domains** with their discovered nameservers.
+- **Hostnames** with per-nameserver DNS records and status.
+- **Ports** with open/closed state and associated hostnames.
+- **TLS certificates** with CN, issuer, expiry, and status.
+- **Recent alerts** (last 100 notifications sent since the process
+  started), displayed in reverse chronological order.
+
+Every data point shows its age (e.g. "5m ago") so you can tell at a
+glance how fresh the information is. The page auto-refreshes every 30
+seconds.
+
+The dashboard intentionally does not expose any configuration details
+such as webhook URLs, notification endpoints, or API tokens.
+
 ### HTTP API
 
 dnswatcher exposes a lightweight HTTP API for operational visibility:
 
 | Endpoint                              | Description                    |
 |---------------------------------------|--------------------------------|
+| `GET /`                               | Web dashboard (HTML)           |
 | `GET /health`                         | Health check (JSON)            |
 | `GET /api/v1/status`                  | Current monitoring state       |
 | `GET /metrics`                        | Prometheus metrics (optional)  |