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/server.go

@@ -0,0 +1,28 @@
+package model
+
+import (
+	"time"
+)
+
+// Server model
+type Server struct {
+	KeyPair   *ServerKeypair
+	Interface *ServerInterface
+}
+
+// ServerKeypair model
+type ServerKeypair struct {
+	PrivateKey string    `json:"private_key"`
+	PublicKey  string    `json:"public_key"`
+	UpdatedAt  time.Time `json:"updated_at"`
+}
+
+// ServerInterface model
+type ServerInterface struct {
+	Addresses  []string  `json:"addresses"`
+	ListenPort int       `json:"listen_port,string"` // ,string to get listen_port string input as int
+	UpdatedAt  time.Time `json:"updated_at"`
+	PostUp     string    `json:"post_up"`
+	PreDown    string    `json:"pre_down"`
+	PostDown   string    `json:"post_down"`
+}