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/lib/comparison-api.ts

@@ -0,0 +1,15 @@
+import { listingsApi, type ListingDetail } from './listings-api';
+
+/**
+ * Fetch multiple listing details in parallel for comparison.
+ * Returns only successfully fetched listings (silently skips 404s).
+ */
+export async function fetchListingsForComparison(
+  ids: string[],
+): Promise<ListingDetail[]> {
+  const results = await Promise.allSettled(ids.map((id) => listingsApi.getById(id)));
+
+  return results
+    .filter((r): r is PromiseFulfilledResult<ListingDetail> => r.status === 'fulfilled')
+    .map((r) => r.value);
+}