fix(docker): pin frontend stage to BUILDPLATFORM and drop removed buildx input

node:22-alpine has no manifest for linux/arm/v6, breaking multi-arch
builds. Frontend output is static JS/CSS that doesn't need to be
built per target arch — pin the stage to $BUILDPLATFORM so Vite
always runs on the host. Also drop `install: true` from
setup-buildx-action@v4 (input was removed).

Dockerfile

@@ -1,7 +1,7 @@
 # ========================================================
 # Stage: Frontend (Vite)
 # ========================================================
-FROM node:22-alpine AS frontend
+FROM --platform=$BUILDPLATFORM node:22-alpine AS frontend
 WORKDIR /src/frontend
 COPY frontend/package.json frontend/package-lock.json ./
 RUN npm ci