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/listings/listing-detail-client.tsx

@@ -3,6 +3,7 @@
 import dynamic from 'next/dynamic';
 import Link from 'next/link';
 import * as React from 'react';
+import { AddToCompareButton } from '@/components/comparison/add-to-compare-button';
 import { ImageGallery } from '@/components/listings/image-gallery';
 import { Badge } from '@/components/ui/badge';
 import { Button } from '@/components/ui/button';
@@ -81,6 +82,9 @@ export function ListingDetailClient({ listing }: ListingDetailClientProps) {
               Thu\u00ea: {formatPrice(listing.rentPriceMonthly)}/th\u00e1ng
             </p>
           )}
+          <div className="mt-3">
+            <AddToCompareButton listingId={listing.id} />
+          </div>
         </div>
       </div>