chore(260329-x2a): add Dockerfile and .dockerignore for containerized deployment

- Multi-stage build: node:22-alpine for build, separate stage for serve
- Runs vite preview on port 4173 with host 0.0.0.0
- config.json mountable at runtime via -v /path/to/config.json:/app/dist/config.json:ro

.dockerignore

@@ -0,0 +1,8 @@
+node_modules
+dist
+.git
+.planning
+*.md
+.biome
+.claude
+.agents

Dockerfile

@@ -0,0 +1,26 @@
+# Stage 1 — build
+FROM node:22-alpine AS build
+
+WORKDIR /app
+
+COPY package*.json ./
+RUN npm ci
+
+COPY . .
+RUN npm run build
+
+# Stage 2 — serve
+FROM node:22-alpine AS serve
+
+WORKDIR /app
+
+COPY --from=build /app/package*.json ./
+RUN npm ci
+
+COPY --from=build /app/dist ./dist
+COPY --from=build /app/vite.config.ts ./vite.config.ts
+
+EXPOSE 4173
+
+# Mount config: docker run -v /path/to/config.json:/app/dist/config.json:ro -p 4173:4173 zauberwald
+CMD ["npm", "run", "preview"]