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>

model/client.go

@@ -0,0 +1,38 @@
+package model
+
+import (
+	"time"
+)
+
+// Client model
+type Client struct {
+	ID              string    `json:"id"`
+	PrivateKey      string    `json:"private_key"`
+	PublicKey       string    `json:"public_key"`
+	PresharedKey    string    `json:"preshared_key"`
+	Name            string    `json:"name"`
+	TgUserid        string    `json:"telegram_userid"`
+	Email           string    `json:"email"`
+	SubnetRanges    []string  `json:"subnet_ranges,omitempty"`
+	AllocatedIPs    []string  `json:"allocated_ips"`
+	AllowedIPs      []string  `json:"allowed_ips"`
+	ExtraAllowedIPs []string  `json:"extra_allowed_ips"`
+	Endpoint        string    `json:"endpoint"`
+	AdditionalNotes string    `json:"additional_notes"`
+	UseServerDNS    bool      `json:"use_server_dns"`
+	Enabled         bool      `json:"enabled"`
+	CreatedAt       time.Time `json:"created_at"`
+	UpdatedAt       time.Time `json:"updated_at"`
+}
+
+// ClientData includes the Client and extra data
+type ClientData struct {
+	Client *Client
+	QRCode string
+}
+
+type QRCodeSettings struct {
+	Enabled    bool
+	IncludeDNS bool
+	IncludeMTU bool
+}