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>

stores/hgspot.yaml

@@ -0,0 +1,14 @@
+name: HG Spot
+base_url: https://www.hgspot.hr
+search_url: https://www.hgspot.hr/pretraga?q={query}&page=0
+category: Electronics
+currency: EUR
+render_js: true
+rate_limit: 1
+
+selectors:
+  container: "section.product-container-comm"
+  name: "h2.product-name"
+  price: "div.product-price"
+  link: "h2.product-name a"
+  image: ".product-image img"