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>

systemd/wireguard-ui-multi.service

@@ -1,22 +0,0 @@
-[Unit]
-Description=wireguard-ui-multi - native multi-server WireGuard management UI
-After=network-online.target
-Wants=network-online.target
-
-[Service]
-Type=simple
-# Runs as root because it shells out to wg-quick, systemctl and nft, which
-# require CAP_NET_ADMIN (and in practice broad privileges for systemctl unit
-# management). AmbientCapabilities is set as defense-in-depth in case this
-# unit is ever adapted to run as a non-root user with File capabilities on
-# the binary instead.
-User=root
-Group=root
-AmbientCapabilities=CAP_NET_ADMIN
-ExecStart=/usr/local/bin/wireguard-ui-multi --db /var/lib/wireguard-ui-multi/wireguard-ui-multi.db --config-dir /etc/wireguard --hooks-dir /etc/wireguard-manager/hooks
-Restart=on-failure
-RestartSec=5
-WorkingDirectory=/var/lib/wireguard-ui-multi
-
-[Install]
-WantedBy=multi-user.target