refactor: configuration into separate file

config.go

@@ -0,0 +1,46 @@
+package smtp2shoutrrr
+
+import (
+	"log/slog"
+	"net/url"
+)
+
+type Config struct {
+	Port       int
+	Username   string
+	Password   string
+	Recipients []ConfigRecipient
+}
+
+func (c *Config) SetDefaults() {
+	if c.Port == 0 {
+		c.Port = 11125
+	}
+
+	if c.Username == "" {
+		slog.Warn("no username provided, using default: username")
+		c.Username = "username"
+	}
+
+	if c.Password == "" {
+		slog.Warn("no password provided, using default: password")
+		c.Password = "password"
+	}
+}
+
+type ConfigRecipient struct {
+	Addresses []string // email addresses
+	Target    string   // shoutrrr address
+	targetURL *url.URL
+}
+
+func (cr ConfigRecipient) GetTargetURL() *url.URL {
+	if cr.targetURL == nil {
+		var err error
+		cr.targetURL, err = url.Parse(cr.Target)
+		if err != nil {
+			slog.Error("failed to parse shoutrrr target URL", slog.String("target", cr.Target), slog.String("err", err.Error()))
+		}
+	}
+	return cr.targetURL
+}