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>

examples/docker-compose/system.yml

@@ -0,0 +1,27 @@
+version: "3"
+
+services:
+  wireguard-ui:
+    image: ngoduykhanh/wireguard-ui:latest
+    container_name: wireguard-ui
+    cap_add:
+      - NET_ADMIN
+    # required to show active clients. with this set, you don't need to expose the ui port (5000) anymore
+    network_mode: host
+    environment:
+      - SENDGRID_API_KEY
+      - EMAIL_FROM_ADDRESS
+      - EMAIL_FROM_NAME
+      - SESSION_SECRET
+      - WGUI_USERNAME=admin
+      - WGUI_PASSWORD=admin
+      - WG_CONF_TEMPLATE
+      - WGUI_MANAGE_START=false
+      - WGUI_MANAGE_RESTART=false
+    logging:
+      driver: json-file
+      options:
+        max-size: 50m
+    volumes:
+      - ./db:/app/db
+      - /etc/wireguard:/etc/wireguard