fix: stop daemon restart loop on port conflict

[structName]

Summary

• map sbm Web port bind conflicts to a dedicated exit code
• prevent systemd from restarting sbm on that expected port-conflict exit
• keep normal daemon keepalive and sing-box desired-state recovery behavior unchanged

Tests

• pnpm install --frozen-lockfile
• pnpm run build
• go test ./...
• CGO_ENABLED=0 go build -ldflags "-s -w -X main.Version=0.2.18-portfix" -o dist/sbm-linux-amd64 ./cmd/sbm/