fix(docker): pin frontend-build to $BUILDPLATFORM to avoid QEMU V8 crash

Multi-arch image builds (linux/amd64,linux/arm64) emulate the foreign arch via
QEMU. The unpinned `FROM node:22-slim` frontend-build stage was built per
target arch, so Node ran under emulation and V8's baseline JIT crashed (SIGTRAP
/ exit 133 on `npm ci`) on the CI runner's QEMU.

Pin the stage to --platform=$BUILDPLATFORM so the static, arch-independent SPA
dist/ is built once on the native build host and COPYd into each arch's runtime
stage. Node never runs emulated; both arch images are still produced.

Dockerfile

@@ -1,5 +1,10 @@
-# Stage 1: build the React SPA
-FROM node:22-slim AS frontend-build
+# Stage 1: build the React SPA.
+# Pin to the native build host ($BUILDPLATFORM) so the Node/V8 build never runs
+# under QEMU during multi-arch builds — emulated Node crashes V8's baseline JIT
+# (SIGTRAP / exit 133 on `npm ci`). The dist/ output is static JS/CSS, i.e.
+# architecture-independent, so building it once and COPYing it into each
+# target-arch runtime stage is both correct and avoids the emulator entirely.
+FROM --platform=$BUILDPLATFORM node:22-slim AS frontend-build
 
 WORKDIR /frontend