Fix 404s: serve UI templates/static from configurable ui-root, not CWD

The web UI 404'd in production because templates/static were loaded
via relative paths ("internal/ui/templates", "internal/ui/static"),
which only resolved when running from the repo checkout. systemd sets
WorkingDirectory=/var/lib/wireguard-ui-multi, so those paths never
existed there.

Add a -ui-root flag (default /usr/local/share/wireguard-ui-multi/ui),
have install.sh copy internal/ui there, and resolve templates/static
paths through it instead of hardcoded relative strings.

Also add release-binary fast path to bootstrap.sh (falls back to
source build with CGO_ENABLED=0/-trimpath if no release exists yet),
and document real hardware/build-RAM requirements in the README.

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

update.sh

@@ -53,7 +53,7 @@ fi
 echo "Rebuilding..."
 cd "$SRC_DIR"
 go mod tidy
-go build -o wireguard-ui-multi ./cmd/wireguard-ui-multi
+CGO_ENABLED=0 go build -trimpath -ldflags="-s -w" -o wireguard-ui-multi ./cmd/wireguard-ui-multi
 
 echo "Reinstalling..."
 bash scripts/install.sh