feat: add React frontend - homepage, auth, products, craftsmen pages

- React 18 + Vite + TypeScript + TailwindCSS + shadcn/ui components
- Auth pages (login/register) with JWT token management via Zustand store
- Homepage with 9 craft category tiles (ceramics first, lacquerware second)
- METI 伝統的工芸品 badge and featured section on homepage
- Products page with category filters + METI認定 filter
- Craftsmen list page with METI badge display
- Navbar with auth-aware navigation
- Japanese warm color theme (amber/terracotta)
- API proxy config pointing to Express backend

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

Dockerfile

@@ -0,0 +1,17 @@
+# Stage 1: Build frontend
+FROM node:20-alpine AS frontend-builder
+WORKDIR /app/client
+COPY client/package*.json ./
+RUN npm install
+COPY client/ ./
+RUN npm run build
+
+# Stage 2: Production
+FROM node:20-alpine
+WORKDIR /app
+COPY package*.json ./
+RUN npm install --production
+COPY src/ ./src/
+COPY --from=frontend-builder /app/client/dist ./client/dist
+EXPOSE 3000
+CMD ["node", "server.js"]