Run go mod tidy before build in all installers

go.sum was never committed (no local Go toolchain to generate it),
causing "missing go.sum entry" build failures on target hosts.
go mod tidy regenerates it automatically before go build.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

scripts/proxmox-install.sh

@@ -161,6 +161,7 @@ echo "Running native installer inside container..."
 pct exec "$VMID" -- bash -c "
 	set -e
 	cd /opt/wireguard-ui-multi-src
+	go mod tidy
 	go build -o wireguard-ui-multi ./cmd/wireguard-ui-multi
 	bash scripts/install.sh
 "