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/src/App.jsx

@@ -3,6 +3,7 @@ import { BrowserRouter as Router, Routes, Route } from 'react-router-dom'
 import { Toaster } from 'sonner'
 import Layout from '@/components/layout/Layout'
 import Dashboard from '@/pages/Dashboard'
+import Search from '@/pages/Search'
 import Settings from '@/pages/Settings'
 import { useTheme } from '@/hooks/use-theme'
 
@@ -15,6 +16,7 @@ export default function App() {
             <Layout theme={theme} toggleTheme={toggleTheme}>
                 <Routes>
                     <Route path="/" element={<Dashboard />} />
+                    <Route path="/search" element={<Search />} />
                     <Route path="/settings" element={<Settings />} />
                 </Routes>
             </Layout>