feat(web): add property comparison page with side-by-side view

Build a complete property comparison feature at /compare:
- Zustand store with localStorage persistence for selected listings (2-5)
- Side-by-side comparison table (price, area, price/m², amenities, location, etc.)
- Summary statistics banner (price range, area range, price/m² range)
- "Add to Compare" button on property cards and detail pages
- Floating comparison bar for quick access when listings are selected
- Bilingual i18n support (Vietnamese + English)
- 18 unit tests for store logic and comparison stats computation
- Mobile-responsive layout with horizontal scroll on comparison table

Co-Authored-By: Paperclip <noreply@paperclip.ing>

apps/web/components/search/property-card.tsx

@@ -1,5 +1,6 @@
 import Image from 'next/image';
 import Link from 'next/link';
+import { AddToCompareButton } from '@/components/comparison/add-to-compare-button';
 import { Badge } from '@/components/ui/badge';
 import { Card, CardContent } from '@/components/ui/card';
 import { formatPrice } from '@/lib/currency';
@@ -58,6 +59,9 @@ export function PropertyCard({ listing, compact }: PropertyCardProps) {
                 </Badge>
               </div>
             )}
+            <div className="absolute right-2 top-2">
+              <AddToCompareButton listingId={listing.id} compact />
+            </div>
           </div>
           <CardContent className="p-4">
             <p className="text-lg font-bold text-primary">