feat: add unauthenticated web dashboard showing monitoring state and recent alerts (#83)

## Summary

Adds a read-only web dashboard at `GET /` that shows the current monitoring state and recent alerts. Unauthenticated, single-page, no navigation.

## What it shows

- **Summary bar**: counts of monitored domains, hostnames, ports, certificates
- **Domains**: nameservers with last-checked age
- **Hostnames**: per-nameserver DNS records, status badges, relative age
- **Ports**: open/closed state with associated hostnames and age
- **TLS Certificates**: CN, issuer, expiry (color-coded by urgency), status, age
- **Recent Alerts**: last 100 notifications in reverse chronological order with priority badges

Every data point displays its age (e.g. "5m ago") so freshness is visible at a glance. Auto-refreshes every 30 seconds.

## What it does NOT show

No secrets: webhook URLs, ntfy topics, Slack/Mattermost endpoints, API tokens, and configuration details are never exposed.

## Design

All assets (CSS) are embedded in the binary and served from `/s/`. Zero external HTTP requests at runtime — no CDN dependencies or third-party resources. Dark, technical aesthetic with saturated teals and blues on dark slate. Single page — everything on one screen.

## Implementation

- `internal/notify/history.go` — thread-safe ring buffer (`AlertHistory`) storing last 100 alerts
- `internal/notify/notify.go` — records each alert in history before dispatch; refactored `SendNotification` into smaller `dispatch*` helpers to satisfy funlen
- `internal/handlers/dashboard.go` — `HandleDashboard()` handler with embedded HTML template, helper functions (`relTime`, `formatRecords`, `expiryDays`, `joinStrings`)
- `internal/handlers/templates/dashboard.html` — Tailwind-styled single-page dashboard
- `internal/handlers/handlers.go` — added `State` and `Notify` dependencies via fx
- `internal/server/routes.go` — registered `GET /` route
- `static/` — embedded CSS assets served via `/s/` prefix
- `README.md` — documented the dashboard and new endpoint

## Tests

- `internal/notify/history_test.go` — empty, add+recent ordering, overflow beyond capacity
- `internal/handlers/dashboard_test.go` — `relTime`, `expiryDays`, `formatRecords`
- All existing tests pass unchanged
- `docker build .` passes

closes [#82](#82)

<!-- session: rework-pr-83 -->

Co-authored-by: user <user@Mac.lan guest wan>
Co-authored-by: clawbot <clawbot@noreply.git.eeqj.de>
Reviewed-on: #83
Co-authored-by: clawbot <clawbot@noreply.example.org>
Co-committed-by: clawbot <clawbot@noreply.example.org>

internal/notify/notify.go

@@ -112,6 +112,7 @@ type Service struct {
 	ntfyURL              *url.URL
 	slackWebhookURL      *url.URL
 	mattermostWebhookURL *url.URL
+	history              *AlertHistory
 }
 
 // New creates a new notify Service.
@@ -123,6 +124,7 @@ func New(
 		log:       params.Logger.Get(),
 		transport: http.DefaultTransport,
 		config:    params.Config,
+		history:   NewAlertHistory(),
 	}
 
 	if params.Config.NtfyTopic != "" {
@@ -167,65 +169,99 @@ func New(
 	return svc, nil
 }
 
+// History returns the alert history for reading recent alerts.
+func (svc *Service) History() *AlertHistory {
+	return svc.history
+}
+
 // SendNotification sends a notification to all configured
-// endpoints.
+// endpoints and records it in the alert history.
 func (svc *Service) SendNotification(
 	ctx context.Context,
 	title, message, priority string,
 ) {
-	if svc.ntfyURL != nil {
-		go func() {
-			notifyCtx := context.WithoutCancel(ctx)
+	svc.history.Add(AlertEntry{
+		Timestamp: time.Now().UTC(),
+		Title:     title,
+		Message:   message,
+		Priority:  priority,
+	})
 
-			err := svc.sendNtfy(
-				notifyCtx,
-				svc.ntfyURL,
-				title, message, priority,
-			)
-			if err != nil {
-				svc.log.Error(
-					"failed to send ntfy notification",
-					"error", err,
-				)
-			}
-		}()
+	svc.dispatchNtfy(ctx, title, message, priority)
+	svc.dispatchSlack(ctx, title, message, priority)
+	svc.dispatchMattermost(ctx, title, message, priority)
+}
+
+func (svc *Service) dispatchNtfy(
+	ctx context.Context,
+	title, message, priority string,
+) {
+	if svc.ntfyURL == nil {
+		return
 	}
 
-	if svc.slackWebhookURL != nil {
-		go func() {
-			notifyCtx := context.WithoutCancel(ctx)
+	go func() {
+		notifyCtx := context.WithoutCancel(ctx)
 
-			err := svc.sendSlack(
-				notifyCtx,
-				svc.slackWebhookURL,
-				title, message, priority,
+		err := svc.sendNtfy(
+			notifyCtx, svc.ntfyURL,
+			title, message, priority,
+		)
+		if err != nil {
+			svc.log.Error(
+				"failed to send ntfy notification",
+				"error", err,
 			)
-			if err != nil {
-				svc.log.Error(
-					"failed to send slack notification",
-					"error", err,
-				)
-			}
-		}()
+		}
+	}()
+}
+
+func (svc *Service) dispatchSlack(
+	ctx context.Context,
+	title, message, priority string,
+) {
+	if svc.slackWebhookURL == nil {
+		return
 	}
 
-	if svc.mattermostWebhookURL != nil {
-		go func() {
-			notifyCtx := context.WithoutCancel(ctx)
+	go func() {
+		notifyCtx := context.WithoutCancel(ctx)
 
-			err := svc.sendSlack(
-				notifyCtx,
-				svc.mattermostWebhookURL,
-				title, message, priority,
+		err := svc.sendSlack(
+			notifyCtx, svc.slackWebhookURL,
+			title, message, priority,
+		)
+		if err != nil {
+			svc.log.Error(
+				"failed to send slack notification",
+				"error", err,
 			)
-			if err != nil {
-				svc.log.Error(
-					"failed to send mattermost notification",
-					"error", err,
-				)
-			}
-		}()
+		}
+	}()
+}
+
+func (svc *Service) dispatchMattermost(
+	ctx context.Context,
+	title, message, priority string,
+) {
+	if svc.mattermostWebhookURL == nil {
+		return
 	}
+
+	go func() {
+		notifyCtx := context.WithoutCancel(ctx)
+
+		err := svc.sendSlack(
+			notifyCtx, svc.mattermostWebhookURL,
+			title, message, priority,
+		)
+		if err != nil {
+			svc.log.Error(
+				"failed to send mattermost notification",
+				"error", err,
+			)
+		}
+	}()
 }
 
 func (svc *Service) sendNtfy(