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/handlers.go

@@ -11,6 +11,8 @@ import (
 	"sneak.berlin/go/dnswatcher/internal/globals"
 	"sneak.berlin/go/dnswatcher/internal/healthcheck"
 	"sneak.berlin/go/dnswatcher/internal/logger"
+	"sneak.berlin/go/dnswatcher/internal/notify"
+	"sneak.berlin/go/dnswatcher/internal/state"
 )
 
 // Params contains dependencies for Handlers.
@@ -20,23 +22,29 @@ type Params struct {
 	Logger      *logger.Logger
 	Globals     *globals.Globals
 	Healthcheck *healthcheck.Healthcheck
+	State       *state.State
+	Notify      *notify.Service
 }
 
 // Handlers provides HTTP request handlers.
 type Handlers struct {
-	log     *slog.Logger
-	params  *Params
-	globals *globals.Globals
-	hc      *healthcheck.Healthcheck
+	log           *slog.Logger
+	params        *Params
+	globals       *globals.Globals
+	hc            *healthcheck.Healthcheck
+	state         *state.State
+	notifyHistory *notify.AlertHistory
 }
 
 // New creates a new Handlers instance.
 func New(_ fx.Lifecycle, params Params) (*Handlers, error) {
 	return &Handlers{
-		log:     params.Logger.Get(),
-		params:  &params,
-		globals: params.Globals,
-		hc:      params.Healthcheck,
+		log:           params.Logger.Get(),
+		params:        &params,
+		globals:       params.Globals,
+		hc:            params.Healthcheck,
+		state:         params.State,
+		notifyHistory: params.Notify.History(),
 	}, nil
 }