Add Puppeteer browser scraping and HG Spot store config

- Add browser-scraper.ts using Puppeteer for JS-heavy stores
- Add render_js flag to store model, migration, YAML sync, and UI
- Scraper engine auto-selects cheerio vs Puppeteer based on flag
- Store forms include JS rendering toggle in Advanced section
- Create first store config: HG Spot (Croatian electronics retailer)
- Update Dockerfile with Chromium for production Puppeteer support

Tested: HG Spot returns 15 products per page with correct names,
prices (EUR), links, and images using headless browser rendering.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Dockerfile

@@ -19,6 +19,11 @@ RUN npx tsc
 FROM node:20-alpine
 WORKDIR /app
 
+# Install Chromium for Puppeteer
+RUN apk add --no-cache chromium
+ENV PUPPETEER_EXECUTABLE_PATH=/usr/bin/chromium-browser
+ENV PUPPETEER_SKIP_CHROMIUM_DOWNLOAD=true
+
 COPY package*.json ./
 RUN npm ci --omit=dev
 
@@ -29,6 +34,7 @@ COPY src/server/db/migrations ./dist/server/db/migrations
 ENV NODE_ENV=production
 ENV PORT=3000
 ENV DATABASE_PATH=/app/data/pricehunter.db
+ENV STORES_DIR=/app/stores
 
 EXPOSE 3000
 VOLUME /app/data