feat(web): centralise Vietnamese price formatting across all pages

Create a single `currency.ts` utility with `formatPrice`, `formatVND`,
`formatPricePerM2`, and `parseVND` to replace 9+ duplicated inline
formatters. This fixes inconsistent decimal handling (1.5M was truncated
to "1 triệu") and standardises price/m² display. Integrated across
property cards, listing detail, dashboard, analytics, payments, pricing,
and admin moderation pages with 19 new unit tests.

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

apps/web/app/[locale]/(dashboard)/listings/page.tsx

@@ -8,15 +8,10 @@ import { Badge } from '@/components/ui/badge';
 import { Button } from '@/components/ui/button';
 import { Card, CardContent, CardDescription, CardHeader, CardTitle } from '@/components/ui/card';
 import { Select } from '@/components/ui/select';
+import { formatPrice } from '@/lib/currency';
 import { useListingsSearch } from '@/lib/hooks/use-listings';
 import type { ListingDetail as _ListingDetail } from '@/lib/listings-api';
 import { PROPERTY_TYPES, TRANSACTION_TYPES, LISTING_STATUSES } from '@/lib/validations/listings';
-function formatPrice(priceVND: string): string {
-  const num = Number(priceVND);
-  if (num >= 1_000_000_000) return `${(num / 1_000_000_000).toFixed(1)} tỷ`;
-  if (num >= 1_000_000) return `${(num / 1_000_000).toFixed(0)} triệu`;
-  return num.toLocaleString('vi-VN');
-}
 
 function formatDate(dateStr: string | null): string {
   if (!dateStr) return 'N/A';
@@ -75,7 +70,7 @@ export default function ListingsPage() {
       </div>
 
       {/* Stats */}
-      <div className="grid gap-3 sm:grid-cols-4">
+      <div className="grid grid-cols-2 gap-3 sm:grid-cols-4">
         <Card>
           <CardHeader className="pb-2">
             <CardDescription>Tổng tin đăng</CardDescription>