build(docker): add multi-stage build for SvelteKit dashboard

Dockerfile

@@ -1,3 +1,15 @@
+# ── Stage 1: build the SvelteKit dashboard ──
+FROM node:24-slim AS dashboard-build
+
+WORKDIR /dashboard
+
+COPY dashboard/package.json dashboard/package-lock.json ./
+RUN npm ci
+
+COPY dashboard/ ./
+RUN npm run build
+
+# ── Stage 2: Python runtime ──
 FROM python:3.12-slim
 
 WORKDIR /app
@@ -9,6 +21,10 @@ RUN apt-get update \
 COPY . .
 RUN pip install --no-cache-dir .
 
+# dashboard/build is .dockerignored from the host context; copy the freshly
+# built dashboard from stage 1 so FastAPI can serve it at /app/dashboard/build
+COPY --from=dashboard-build /dashboard/build ./dashboard/build
+
 EXPOSE 8000
 
 CMD ["python", "run.py"]