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/interface.go

@@ -0,0 +1,10 @@
+package emailer
+
+type Attachment struct {
+	Name string
+	Data []byte
+}
+
+type Emailer interface {
+	Send(toName string, to string, subject string, content string, attachments []Attachment) error
+}

emailer/sendgrid.go

@@ -0,0 +1,54 @@
+package emailer
+
+import (
+	"encoding/base64"
+
+	"github.com/sendgrid/sendgrid-go"
+	"github.com/sendgrid/sendgrid-go/helpers/mail"
+)
+
+type SendgridApiMail struct {
+	apiKey   string
+	fromName string
+	from     string
+}
+
+func NewSendgridApiMail(apiKey, fromName, from string) *SendgridApiMail {
+	ans := SendgridApiMail{apiKey: apiKey, fromName: fromName, from: from}
+	return &ans
+}
+
+func (o *SendgridApiMail) Send(toName string, to string, subject string, content string, attachments []Attachment) error {
+	m := mail.NewV3Mail()
+
+	mailFrom := mail.NewEmail(o.fromName, o.from)
+	mailContent := mail.NewContent("text/html", content)
+	mailTo := mail.NewEmail(toName, to)
+
+	m.SetFrom(mailFrom)
+	m.AddContent(mailContent)
+
+	personalization := mail.NewPersonalization()
+	personalization.AddTos(mailTo)
+	personalization.Subject = subject
+
+	m.AddPersonalizations(personalization)
+
+	toAdd := make([]*mail.Attachment, 0, len(attachments))
+	for i := range attachments {
+		var att mail.Attachment
+		encoded := base64.StdEncoding.EncodeToString(attachments[i].Data)
+		att.SetContent(encoded)
+		att.SetType("text/plain")
+		att.SetFilename(attachments[i].Name)
+		att.SetDisposition("attachment")
+		toAdd = append(toAdd, &att)
+	}
+
+	m.AddAttachment(toAdd...)
+	request := sendgrid.GetRequest(o.apiKey, "/v3/mail/send", "https://api.sendgrid.com")
+	request.Method = "POST"
+	request.Body = mail.GetRequestBody(m)
+	_, err := sendgrid.API(request)
+	return err
+}

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
+}