client: chirp offer every 5s while unpaired

Solves the late-subscriber problem — browsers that load the page after
the peer's startup offer would never see one. Now the peer re-broadcasts
the offer every 5 seconds until the DataChannel opens, then stops.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

client/main.go

@@ -21,6 +21,7 @@ import (
 	"os"
 	"runtime"
 	"strings"
+	"sync"
 	"time"
 
 	"github.com/atotto/clipboard"
@@ -179,7 +180,10 @@ func handleMessage(ev string, m Message) {
 
 // ── WebRTC ────────────────────────────────────────────────────────────────
 
-var incomingSignal = make(chan Message, 16)
+var (
+	incomingSignal = make(chan Message, 16)
+	rtcConnected   = make(chan struct{})
+)
 
 func runRTCPeer(server string) {
 	api := webrtc.NewAPI()
@@ -200,8 +204,10 @@ func runRTCPeer(server string) {
 		return
 	}
 
+	var once sync.Once
 	dc.OnOpen(func() {
 		fmt.Fprintln(os.Stderr, "rtc: DataChannel OPEN — P2P live")
+		once.Do(func() { close(rtcConnected) })
 	})
 	dc.OnMessage(func(msg webrtc.DataChannelMessage) {
 		text := string(msg.Data)
@@ -225,7 +231,7 @@ func runRTCPeer(server string) {
 		fmt.Fprintf(os.Stderr, "rtc: state=%s\n", s)
 	})
 
-	// Create offer + post via signaling bus
+	// Create offer + post once immediately
 	offer, err := pc.CreateOffer(nil)
 	if err != nil {
 		log.Printf("rtc: create offer: %v", err)
@@ -237,7 +243,24 @@ func runRTCPeer(server string) {
 	}
 	payload, _ := json.Marshal(SignalPayload{Kind: "offer", SDP: &offer})
 	send(server, myLabel, "", "signal", payload)
-	fmt.Fprintln(os.Stderr, "rtc: offer posted, waiting for answer...")
+	fmt.Fprintln(os.Stderr, "rtc: offer posted, will chirp every 5s until paired...")
+
+	// "Chirp": re-post the offer every 5s until DataChannel opens. Catches
+	// late-joining browsers without needing server-side history of signals.
+	go func() {
+		t := time.NewTicker(5 * time.Second)
+		defer t.Stop()
+		for {
+			select {
+			case <-rtcConnected:
+				fmt.Fprintln(os.Stderr, "rtc: paired — chirping stopped")
+				return
+			case <-t.C:
+				p, _ := json.Marshal(SignalPayload{Kind: "offer", SDP: &offer})
+				send(server, myLabel, "", "signal", p)
+			}
+		}
+	}()
 
 	// Process incoming signals
 	for {