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>

package.json

@@ -22,6 +22,7 @@
     "dotenv": "^16.4.7",
     "fastify": "^5.2.1",
     "p-limit": "^6.2.0",
+    "puppeteer": "^24.40.0",
     "sql.js": "^1.11.0",
     "yaml": "^2.8.3"
   },