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/lib/currency.ts

@@ -0,0 +1,121 @@
+/**
+ * Vietnamese currency formatting utilities.
+ *
+ * Centralised formatter for all price displays across the platform.
+ * Converts raw VND numbers into human-readable Vietnamese format:
+ *   3,500,000,000 -> "3.5 ty"
+ *   150,000,000   -> "150 trieu"
+ *   800,000       -> "800.000"
+ */
+
+// ---------------------------------------------------------------------------
+// Core formatter
+// ---------------------------------------------------------------------------
+
+/**
+ * Format a VND amount into compact Vietnamese notation.
+ *
+ * @example
+ * formatPrice(3_500_000_000) // "3.5 ty"
+ * formatPrice(150_000_000)   // "150 trieu"
+ * formatPrice(1_500_000)     // "1.5 trieu"
+ * formatPrice(800_000)       // "800.000"
+ * formatPrice("3500000000")  // "3.5 ty" (string input accepted)
+ */
+export function formatPrice(amount: string | number): string {
+  const num = typeof amount === 'string' ? Number(amount) : amount;
+  if (!Number.isFinite(num) || num < 0) return '0';
+
+  if (num >= 1_000_000_000) {
+    const billions = num / 1_000_000_000;
+    return `${stripTrailingZero(billions.toFixed(1))} t\u1ef7`;
+  }
+
+  if (num >= 1_000_000) {
+    const millions = num / 1_000_000;
+    return `${stripTrailingZero(millions.toFixed(1))} tri\u1ec7u`;
+  }
+
+  return num.toLocaleString('vi-VN');
+}
+
+// ---------------------------------------------------------------------------
+// Variant: with currency suffix
+// ---------------------------------------------------------------------------
+
+/**
+ * Format a VND amount with a " \u0111" currency suffix.
+ * Returns "Mi\u1ec5n ph\u00ed" for zero amounts.
+ *
+ * @example
+ * formatVND(4_990_000) // "4.99 trieu d"
+ * formatVND(0)         // "Mien phi"
+ */
+export function formatVND(amount: string | number): string {
+  const num = typeof amount === 'string' ? Number(amount) : amount;
+  if (!Number.isFinite(num) || num < 0) return '0 \u0111';
+  if (num === 0) return 'Mi\u1ec5n ph\u00ed';
+
+  if (num >= 1_000_000_000) {
+    const billions = num / 1_000_000_000;
+    return `${stripTrailingZero(billions.toFixed(1))} t\u1ef7 \u0111`;
+  }
+
+  if (num >= 1_000_000) {
+    const millions = num / 1_000_000;
+    return `${stripTrailingZero(millions.toFixed(1))} tri\u1ec7u \u0111`;
+  }
+
+  return num.toLocaleString('vi-VN') + ' \u0111';
+}
+
+// ---------------------------------------------------------------------------
+// Variant: price per square metre
+// ---------------------------------------------------------------------------
+
+/**
+ * Format a VND/m\u00b2 value.
+ *
+ * @example
+ * formatPricePerM2(50_500_000)  // "50.5 tr/m\u00b2"
+ * formatPricePerM2(500_000)     // "500.000 \u0111/m\u00b2"
+ */
+export function formatPricePerM2(price: string | number): string {
+  const num = typeof price === 'string' ? Number(price) : price;
+  if (!Number.isFinite(num) || num < 0) return '0 \u0111/m\u00b2';
+
+  if (num >= 1_000_000_000) {
+    const billions = num / 1_000_000_000;
+    return `${stripTrailingZero(billions.toFixed(1))} t\u1ef7/m\u00b2`;
+  }
+
+  if (num >= 1_000_000) {
+    const millions = num / 1_000_000;
+    return `${stripTrailingZero(millions.toFixed(1))} tr/m\u00b2`;
+  }
+
+  return `${num.toLocaleString('vi-VN')} \u0111/m\u00b2`;
+}
+
+// ---------------------------------------------------------------------------
+// Parser (reverse direction)
+// ---------------------------------------------------------------------------
+
+/**
+ * Parse a formatted Vietnamese price string back into a number.
+ * Returns `null` if the input cannot be parsed.
+ */
+export function parseVND(formatted: string): number | null {
+  const cleaned = formatted.replace(/[^\d]/g, '');
+  if (cleaned === '') return null;
+  return Number(cleaned);
+}
+
+// ---------------------------------------------------------------------------
+// Helpers
+// ---------------------------------------------------------------------------
+
+/** Remove a trailing ".0" so "3.0 ty" becomes "3 ty". */
+function stripTrailingZero(str: string): string {
+  return str.replace(/\.0$/, '');
+}