Replace from-scratch rewrite with real ngoduykhanh/wireguard-ui fork

The from-scratch Go rewrite had unresolved bugs (missing go.sum, UI
404s, path issues) from being built without a working local Go
toolchain to verify against. Switching strategy: use the actual
upstream wireguard-ui codebase (proven, battle-tested single-server
manager) as the base, and extend it for multi-server support instead
of re-deriving everything from zero.

Kept our own installers (bootstrap.sh, update.sh, scripts/install.sh,
scripts/proxmox-install.sh) - these still apply, just need updating
to build/install the upstream module layout instead of the old
cmd/wireguard-ui-multi structure.

Module path intentionally left as upstream's own
(github.com/ngoduykhanh/wireguard-ui) for now to avoid touching every
internal import; revisit if this needs to be fully rebranded.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

emailer/smtp.go

@@ -0,0 +1,100 @@
+package emailer
+
+import (
+	"crypto/tls"
+	"fmt"
+	"strings"
+	"time"
+
+	mail "github.com/xhit/go-simple-mail/v2"
+)
+
+type SmtpMail struct {
+	hostname   string
+	port       int
+	username   string
+	password   string
+	smtpHelo   string
+	authType   mail.AuthType
+	encryption mail.Encryption
+	noTLSCheck bool
+	fromName   string
+	from       string
+}
+
+func authType(authType string) mail.AuthType {
+	switch strings.ToUpper(authType) {
+	case "PLAIN":
+		return mail.AuthPlain
+	case "LOGIN":
+		return mail.AuthLogin
+	default:
+		return mail.AuthNone
+	}
+}
+
+func encryptionType(encryptionType string) mail.Encryption {
+	switch strings.ToUpper(encryptionType) {
+	case "NONE":
+		return mail.EncryptionNone
+	case "SSL":
+		return mail.EncryptionSSL
+	case "SSLTLS":
+		return mail.EncryptionSSLTLS
+	case "TLS":
+		return mail.EncryptionTLS
+	default:
+		return mail.EncryptionSTARTTLS
+	}
+}
+
+func NewSmtpMail(hostname string, port int, username string, password string, SmtpHelo string, noTLSCheck bool, auth string, fromName, from string, encryption string) *SmtpMail {
+	ans := SmtpMail{hostname: hostname, port: port, username: username, password: password, smtpHelo: SmtpHelo, noTLSCheck: noTLSCheck, fromName: fromName, from: from, authType: authType(auth), encryption: encryptionType(encryption)}
+	return &ans
+}
+
+func addressField(address string, name string) string {
+	if name == "" {
+		return address
+	}
+	return fmt.Sprintf("%s <%s>", name, address)
+}
+
+func (o *SmtpMail) Send(toName string, to string, subject string, content string, attachments []Attachment) error {
+	server := mail.NewSMTPClient()
+
+	server.Host = o.hostname
+	server.Port = o.port
+	server.Authentication = o.authType
+	server.Username = o.username
+	server.Password = o.password
+	server.Helo = o.smtpHelo
+	server.Encryption = o.encryption
+	server.KeepAlive = false
+	server.ConnectTimeout = 10 * time.Second
+	server.SendTimeout = 10 * time.Second
+
+	if o.noTLSCheck {
+		server.TLSConfig = &tls.Config{InsecureSkipVerify: true}
+	}
+
+	smtpClient, err := server.Connect()
+
+	if err != nil {
+		return err
+	}
+
+	email := mail.NewMSG()
+	email.SetFrom(addressField(o.from, o.fromName)).
+		AddTo(addressField(to, toName)).
+		SetSubject(subject).
+		SetBody(mail.TextHTML, content)
+
+	for _, v := range attachments {
+		email.Attach(&mail.File{Name: v.Name, Data: v.Data})
+	}
+
+	err = email.Send(smtpClient)
+
+	return err
+}