v0.1: HTTP+SSE broadcast bus + phone web UI + Linux client

Single Go module with two binaries (server, client) and an embedded
phone UI. MVP transport is HTTP POST → SSE fanout; the roadmap calls
for upgrading to WebRTC P2P with Sign in with Apple for identity, mDNS
for discovery, and OS clipboard hooks.

- server/: Go HTTP server, embedded index.html, broadcast bus with
  short replay history, SSE stream endpoint, single-binary deploy.
- client/: subscribes to SSE feed and prints messages; -send for
  one-shot publish from CLI. No OS clipboard touched yet (v0.5).
- web/index.html: dark phone-first UI, paste-clipboard button (uses
  navigator.clipboard.readText), live feed of incoming messages via
  EventSource.

This commit is intentionally tiny — it proves the end-to-end shape so
the WebRTC/SiwA/mDNS pieces can be added incrementally without
restructuring.

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

client/main.go

@@ -0,0 +1,99 @@
+// tether-client: connects to a tether-server, prints messages to stdout.
+// MVP — does not yet touch the OS clipboard.
+package main
+
+import (
+	"bufio"
+	"bytes"
+	"encoding/json"
+	"flag"
+	"fmt"
+	"io"
+	"log"
+	"net/http"
+	"os"
+	"strings"
+	"time"
+)
+
+type Message struct {
+	Text   string `json:"text"`
+	Source string `json:"source,omitempty"`
+	TS     int64  `json:"ts"`
+}
+
+func main() {
+	server := flag.String("server", "http://localhost:8765", "tether-server base URL")
+	label := flag.String("label", "linux-client", "X-Tether-Client label")
+	sendText := flag.String("send", "", "send this text and exit (otherwise listen)")
+	flag.Parse()
+
+	if *sendText != "" {
+		send(*server, *label, *sendText)
+		return
+	}
+
+	for {
+		if err := listen(*server, *label); err != nil {
+			log.Printf("stream error: %v — reconnecting in 3s", err)
+			time.Sleep(3 * time.Second)
+		}
+	}
+}
+
+func send(server, label, text string) {
+	body, _ := json.Marshal(Message{Text: text, Source: label})
+	req, _ := http.NewRequest("POST", server+"/api/send", bytes.NewReader(body))
+	req.Header.Set("Content-Type", "application/json")
+	req.Header.Set("X-Tether-Source", label)
+	r, err := http.DefaultClient.Do(req)
+	if err != nil {
+		log.Fatalf("send: %v", err)
+	}
+	defer r.Body.Close()
+	if r.StatusCode >= 300 {
+		log.Fatalf("send: HTTP %d", r.StatusCode)
+	}
+	fmt.Println("sent.")
+}
+
+func listen(server, label string) error {
+	req, _ := http.NewRequest("GET", server+"/api/stream", nil)
+	req.Header.Set("X-Tether-Client", label)
+	r, err := http.DefaultClient.Do(req)
+	if err != nil {
+		return err
+	}
+	defer r.Body.Close()
+	if r.StatusCode != 200 {
+		return fmt.Errorf("HTTP %d", r.StatusCode)
+	}
+	fmt.Fprintf(os.Stderr, "tether-client: connected to %s as %q\n", server, label)
+	sc := bufio.NewScanner(r.Body)
+	sc.Buffer(make([]byte, 1024*1024), 1024*1024)
+	var ev, data string
+	for sc.Scan() {
+		line := sc.Text()
+		switch {
+		case strings.HasPrefix(line, "event: "):
+			ev = strings.TrimPrefix(line, "event: ")
+		case strings.HasPrefix(line, "data: "):
+			data = strings.TrimPrefix(line, "data: ")
+		case line == "":
+			if ev == "clipboard" && data != "" {
+				var m Message
+				if err := json.Unmarshal([]byte(data), &m); err == nil {
+					ts := time.UnixMilli(m.TS).Format("15:04:05")
+					fmt.Printf("\n──────  %s  from %s  ──────\n%s\n", ts, m.Source, m.Text)
+				}
+			}
+			ev, data = "", ""
+		case strings.HasPrefix(line, ": "):
+			// keepalive — ignore
+		}
+	}
+	if err := sc.Err(); err != nil && err != io.EOF {
+		return err
+	}
+	return nil
+}