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/__tests__/currency.spec.ts

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+import { describe, it, expect } from 'vitest';
+import {
+  formatPrice,
+  formatVND,
+  formatPricePerM2,
+  parseVND,
+} from '../currency';
+
+// ---------------------------------------------------------------------------
+// formatPrice — compact notation without currency suffix
+// ---------------------------------------------------------------------------
+
+describe('formatPrice', () => {
+  it('formats billions as "X ty"', () => {
+    expect(formatPrice(1_000_000_000)).toBe('1 t\u1ef7');
+    expect(formatPrice(1_500_000_000)).toBe('1.5 t\u1ef7');
+    expect(formatPrice(3_500_000_000)).toBe('3.5 t\u1ef7');
+    expect(formatPrice(10_000_000_000)).toBe('10 t\u1ef7');
+  });
+
+  it('formats millions as "X trieu"', () => {
+    expect(formatPrice(1_000_000)).toBe('1 tri\u1ec7u');
+    expect(formatPrice(1_500_000)).toBe('1.5 tri\u1ec7u');
+    expect(formatPrice(150_000_000)).toBe('150 tri\u1ec7u');
+    expect(formatPrice(800_000_000)).toBe('800 tri\u1ec7u');
+    expect(formatPrice(999_000_000)).toBe('999 tri\u1ec7u');
+  });
+
+  it('formats values below 1 million with locale separator', () => {
+    expect(formatPrice(500_000)).toMatch(/500/);
+    expect(formatPrice(1_000)).toMatch(/1/);
+    expect(formatPrice(0)).toBe('0');
+  });
+
+  it('accepts string inputs', () => {
+    expect(formatPrice('3500000000')).toBe('3.5 t\u1ef7');
+    expect(formatPrice('150000000')).toBe('150 tri\u1ec7u');
+  });
+
+  it('handles edge cases gracefully', () => {
+    expect(formatPrice(-1)).toBe('0');
+    expect(formatPrice(NaN)).toBe('0');
+    expect(formatPrice(Infinity)).toBe('0');
+    expect(formatPrice('')).toBe('0');
+  });
+
+  it('strips trailing .0', () => {
+    expect(formatPrice(2_000_000_000)).toBe('2 t\u1ef7');
+    expect(formatPrice(5_000_000)).toBe('5 tri\u1ec7u');
+  });
+});
+
+// ---------------------------------------------------------------------------
+// formatVND — with currency suffix "d"
+// ---------------------------------------------------------------------------
+
+describe('formatVND', () => {
+  it('returns "Mien phi" for zero', () => {
+    expect(formatVND(0)).toBe('Mi\u1ec5n ph\u00ed');
+  });
+
+  it('formats billions with suffix', () => {
+    expect(formatVND(1_500_000_000)).toBe('1.5 t\u1ef7 \u0111');
+  });
+
+  it('formats millions with suffix', () => {
+    expect(formatVND(5_000_000)).toBe('5 tri\u1ec7u \u0111');
+    expect(formatVND(4_990_000)).toBe('5 tri\u1ec7u \u0111');
+  });
+
+  it('formats values below 1 million with suffix', () => {
+    expect(formatVND(500_000)).toMatch(/\u0111$/);
+  });
+
+  it('accepts string input', () => {
+    expect(formatVND('1500000000')).toBe('1.5 t\u1ef7 \u0111');
+  });
+});
+
+// ---------------------------------------------------------------------------
+// formatPricePerM2 — price per square metre
+// ---------------------------------------------------------------------------
+
+describe('formatPricePerM2', () => {
+  it('formats millions as "X tr/m\u00b2"', () => {
+    expect(formatPricePerM2(50_500_000)).toBe('50.5 tr/m\u00b2');
+    expect(formatPricePerM2(1_000_000)).toBe('1 tr/m\u00b2');
+  });
+
+  it('formats billions as "X ty/m\u00b2"', () => {
+    expect(formatPricePerM2(1_500_000_000)).toBe('1.5 t\u1ef7/m\u00b2');
+  });
+
+  it('formats values below 1 million with "/m\u00b2" suffix', () => {
+    expect(formatPricePerM2(500_000)).toMatch(/m\u00b2$/);
+  });
+
+  it('accepts string input', () => {
+    expect(formatPricePerM2('50500000')).toBe('50.5 tr/m\u00b2');
+  });
+
+  it('handles edge cases', () => {
+    expect(formatPricePerM2(0)).toBe('0 \u0111/m\u00b2');
+    expect(formatPricePerM2(-1)).toBe('0 \u0111/m\u00b2');
+  });
+});
+
+// ---------------------------------------------------------------------------
+// parseVND — reverse parse
+// ---------------------------------------------------------------------------
+
+describe('parseVND', () => {
+  it('parses formatted number back', () => {
+    expect(parseVND('500.000')).toBe(500000);
+  });
+
+  it('returns null for empty / non-numeric input', () => {
+    expect(parseVND('')).toBeNull();
+    expect(parseVND('abc')).toBeNull();
+  });
+
+  it('strips non-digit characters', () => {
+    expect(parseVND('1.500.000 \u0111')).toBe(1500000);
+  });
+});

apps/web/lib/currency.ts

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+/**
+ * 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 `${num.toLocaleString('vi-VN')} \u0111/m\u00b2`;
+}
+
+// ---------------------------------------------------------------------------
+// Parser (reverse direction)
+// ---------------------------------------------------------------------------
+
+/**
+ * Parse a formatted Vietnamese price string back into a number.
+ * Returns `null` if the input cannot be parsed.
+ */
+export function parseVND(formatted: string): number | null {
+  const cleaned = formatted.replace(/[^\d]/g, '');
+  if (cleaned === '') return null;
+  return Number(cleaned);
+}
+
+// ---------------------------------------------------------------------------
+// Helpers
+// ---------------------------------------------------------------------------
+
+/** Remove a trailing ".0" so "3.0 ty" becomes "3 ty". */
+function stripTrailingZero(str: string): string {
+  return str.replace(/\.0$/, '');
+}