Listen on 0.0.0.0 by default and log LAN addresses

- New CANON_HOST (default 0.0.0.0; 127.0.0.1 keeps it on one machine)
- On start, log the http:// addresses other devices can use, skipping
  Docker/VM bridge interfaces
- docker-compose publishes 0.0.0.0:4242 explicitly
- README: opening Canon from other devices and firewall note

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

cmd/canon/main.go

@@ -9,6 +9,7 @@ import (
 	"fmt"
 	"log/slog"
 	"math/big"
+	"net"
 	"net/http"
 	"os"
 	"os/signal"
@@ -93,6 +94,7 @@ func run(log *slog.Logger) error {
 	errc := make(chan error, 1)
 	go func() {
 		log.Info("Canon is running", "addr", cfg.ListenAddr, "data", cfg.DataDir, "version", version)
+		logURLs(log, cfg)
 		errc <- httpSrv.ListenAndServe()
 	}()
 	select {
@@ -149,3 +151,49 @@ func setupCode() string {
 	}
 	return string(b[:4]) + "-" + string(b[4:])
 }
+
+// logURLs prints the addresses other devices can use to reach Canon.
+func logURLs(log *slog.Logger, cfg config.Config) {
+	if cfg.Host != "0.0.0.0" && cfg.Host != "" && cfg.Host != "::" {
+		log.Info("open Canon at", "url", "http://"+net.JoinHostPort(cfg.Host, cfg.Port))
+		return
+	}
+	// Inside Docker the container's own addresses aren't reachable from the
+	// network; the host's address and published port are what matter.
+	if _, err := os.Stat("/.dockerenv"); err == nil {
+		log.Info("listening on all interfaces; open http://<this computer's IP>:<published port> from any device on your network")
+		return
+	}
+	log.Info("open Canon at", "url", "http://localhost:"+cfg.Port)
+	ifaces, err := net.Interfaces()
+	if err != nil {
+		return
+	}
+	for _, iface := range ifaces {
+		// Skip interfaces that are down and container/VM bridges, which
+		// other devices can't reach.
+		if iface.Flags&net.FlagUp == 0 || iface.Flags&net.FlagLoopback != 0 || isVirtualBridge(iface.Name) {
+			continue
+		}
+		addrs, err := iface.Addrs()
+		if err != nil {
+			continue
+		}
+		for _, a := range addrs {
+			ipnet, ok := a.(*net.IPNet)
+			if !ok || ipnet.IP.To4() == nil || ipnet.IP.IsLinkLocalUnicast() {
+				continue
+			}
+			log.Info("open Canon on your network at", "url", "http://"+net.JoinHostPort(ipnet.IP.String(), cfg.Port))
+		}
+	}
+}
+
+func isVirtualBridge(name string) bool {
+	for _, prefix := range []string{"docker", "br-", "veth", "virbr", "cni", "flannel", "podman"} {
+		if strings.HasPrefix(name, prefix) {
+			return true
+		}
+	}
+	return false
+}