feat: add connection status indicator with SSE heartbeat

- Add ConnectionIndicator component showing green/red dot
- Send lastPing signal every 15 seconds via SSE
- Indicator turns red if no ping received in 20 seconds
- Gives users confidence the live connection is active

sse/signals.go

@@ -0,0 +1,28 @@
+// Package sse provides helpers for SSE signal handling.
+package sse
+
+import (
+	"encoding/json"
+	"time"
+
+	"github.com/starfederation/datastar-go/datastar"
+)
+
+// Signals holds client-side state managed via SSE.
+type Signals struct {
+	LastPing int64 `json:"lastPing,omitempty"`
+}
+
+// SendPing sends a heartbeat signal with the current timestamp.
+func SendPing(sse *datastar.ServerSentEventGenerator) error {
+	return PatchSignals(sse, Signals{LastPing: time.Now().UnixMilli()})
+}
+
+// PatchSignals sends a signals patch to the client.
+func PatchSignals(sse *datastar.ServerSentEventGenerator, s Signals) error {
+	data, err := json.Marshal(s)
+	if err != nil {
+		return err
+	}
+	return sse.PatchSignals(data)
+}