Two-stage build: node builds the Svelte SPA to dist, then golang compiles the
binary with the SPA embedded (-tags embedspa). Committed .pb.go stubs mean no
protoc. Ships as a small static distroless/nonroot image; same binary serves the
register-admin subcommand. (Not build-verified here — no docker in the
devcontainer — but the go build -tags embedspa + npm build are both proven.)
