feat(frontend): add product detail view and navigation

- Create ProductView component with product details display
- Add ProductView.css with responsive styling
- Add product screen to App.jsx screen system
- Pass onNavigateToProduct callback from App to SearchView
- Update ProductResults onSelect to use callback instead of window.location

apps/frontend/src/App.jsx

@@ -2,6 +2,7 @@ import React, { useState, useEffect } from 'react';
 import './App.css';
 import HomeView from './views/HomeView';
 import SearchView from './views/SearchView';
+import ProductView from './views/ProductView';
 import ScannerView from './views/ScannerView';
 import AlertsView from './views/AlertsView';
 import ProfileView from './views/ProfileView';
@@ -19,6 +20,7 @@ function App() {
   const [showSaved, setShowSaved] = useState(false);
   const [badgeCount, setBadgeCount] = useState(0);
   const [prescriptionSearch, setPrescriptionSearch] = useState('');
+  const [productScreen, setProductScreen] = useState(null);
   const [screenSize, setScreenSize] = useState({
     width: window.innerWidth,
     height: window.innerHeight
@@ -180,6 +182,16 @@ function App() {
           currentUser={currentUser}
           onLoginRequest={() => setShowLogin(true)}
           initialQuery={prescriptionSearch}
+          onNavigateToProduct={(source, id) => setProductScreen({ source, id })}
+        />
+      );
+      break;
+    case 'product':
+      activeView = (
+        <ProductView
+          source={productScreen?.source}
+          id={productScreen?.id}
+          onBack={() => { setProductScreen(null); setScreen('search'); }}
         />
       );
       break;