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>

internal/config/config.go

@@ -2,6 +2,7 @@
 package config
 
 import (
+	"net"
 	"os"
 	"strconv"
 	"strings"
@@ -12,6 +13,8 @@ import (
 // fields here only act as overrides when set.
 type Config struct {
 	ListenAddr    string
+	Host          string
+	Port          string
 	DataDir       string
 	AdminUsername string
 	AdminPassword string
@@ -28,9 +31,14 @@ type Config struct {
 }
 
 func Load() Config {
+	// 0.0.0.0 accepts connections from other devices on the network, not
+	// just this machine. Use 127.0.0.1 to keep it local.
+	host := env("CANON_HOST", "0.0.0.0")
 	port := env("CANON_PORT", "4242")
 	return Config{
-		ListenAddr:       env("CANON_LISTEN", ":"+port),
+		ListenAddr:       env("CANON_LISTEN", net.JoinHostPort(host, port)),
+		Host:             host,
+		Port:             port,
 		DataDir:          env("CANON_DATA_DIR", "./data"),
 		AdminUsername:    env("CANON_ADMIN_USERNAME", ""),
 		AdminPassword:    env("CANON_ADMIN_PASSWORD", ""),