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/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
+}