feat: Add multi-site product search system with SearXNG integration

This commit adds a comprehensive product search feature that allows users
to search for products across multiple configured e-commerce sites.

Backend changes:
- Add SearXNG service to docker-compose for meta-search capability
- Add SearchSite model and Alembic migration with default sites
- Create searxng_service.py for search queries across sites
- Create light_scraper_service.py for fast HTTP-based extraction
- Create search_service.py orchestrator combining SearXNG + HTTP + Browserless
- Add API endpoints for search (/api/search) with SSE streaming
- Add CRUD endpoints for search sites (/api/search-sites)

Frontend changes:
- Add new /search page with real-time results display
- Create SearchResultCard component for product display
- Add Progress, Checkbox, and Badge UI components
- Update Settings page with search sites management section
- Add navigation link to search page
- Create new modern SVG logo with price-flow gradient design
- Update favicon to SVG format

The search system uses a hybrid approach:
1. SearXNG finds product URLs across configured sites
2. Light HTTP scraper attempts fast extraction
3. Falls back to Browserless for JavaScript-heavy sites
4. Results stream progressively via SSE

frontend/package.json

@@ -9,8 +9,10 @@
         "preview": "vite preview"
     },
     "dependencies": {
+        "@radix-ui/react-checkbox": "^1.1.4",
         "@radix-ui/react-dialog": "^1.1.15",
         "@radix-ui/react-label": "^2.1.8",
+        "@radix-ui/react-progress": "^1.1.4",
         "@radix-ui/react-select": "^2.2.6",
         "@radix-ui/react-slot": "^1.2.4",
         "@radix-ui/react-switch": "^1.2.6",