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/setting.go

@@ -0,0 +1,17 @@
+package model
+
+import (
+	"time"
+)
+
+// GlobalSetting model
+type GlobalSetting struct {
+	EndpointAddress     string    `json:"endpoint_address"`
+	DNSServers          []string  `json:"dns_servers"`
+	MTU                 int       `json:"mtu,string"`
+	PersistentKeepalive int       `json:"persistent_keepalive,string"`
+	FirewallMark        string    `json:"firewall_mark"`
+	Table               string    `json:"table"`
+	ConfigFilePath      string    `json:"config_file_path"`
+	UpdatedAt           time.Time `json:"updated_at"`
+}