fix: build the frontend on the native arch in multi-arch builds

[EdmondDantes21]

The 1.0.28 tag build hung, then died emulating npm install for linux/arm64:

#21 [linux/arm64 react-build 4/7] RUN npm install
#21 69.12 qemu: uncaught target signal 4 (Illegal instruction) - core dumped

react-build produces dist/ — static HTML/JS/CSS with nothing architecture-specific in it — so there is no reason to run Node under QEMU at all. Pinning that stage to $BUILDPLATFORM builds it once on the runner's native architecture; only the final bitnami/nginx stage stays per-platform, and it does nothing but COPY, which never executes target-arch binaries.

This removes emulation from the build entirely and makes it roughly twice as fast.

Verified locally: docker buildx build --platform linux/amd64,linux/arm64 now resolves a single native react-build stage and assembles both images from it.

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