Add `platforms: linux/amd64,linux/arm64` to both the frontend and
backend build-and-push steps. The existing setup-buildx-action already
enables QEMU-based cross-compilation, so no additional steps are needed.
Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>
Former-commit-id: e3e0db6f3d