feat: add fake trading module with crypto, stocks, forex and gold

Paper trading system with 5 commands (trade_topup, trade_buy,
trade_sell, trade_convert, trade_stats). Supports VN stocks via TCBS,
crypto via CoinGecko, forex via ER-API, and gold via PAX Gold proxy.
Per-user portfolio stored in KV with 60s price caching. 54 new tests.

tests/modules/trading/format.test.js

@@ -0,0 +1,79 @@
+import { describe, expect, it } from "vitest";
+import {
+  formatCrypto,
+  formatCurrency,
+  formatPnL,
+  formatStock,
+  formatUSD,
+  formatVND,
+} from "../../../src/modules/trading/format.js";
+
+describe("trading/format", () => {
+  describe("formatVND", () => {
+    it("formats with dot thousands separator", () => {
+      expect(formatVND(15000000)).toBe("15.000.000 VND");
+    });
+    it("handles zero", () => {
+      expect(formatVND(0)).toBe("0 VND");
+    });
+    it("handles negative", () => {
+      expect(formatVND(-1500000)).toBe("-1.500.000 VND");
+    });
+    it("rounds to integer", () => {
+      expect(formatVND(1234.56)).toBe("1.235 VND");
+    });
+    it("handles small numbers", () => {
+      expect(formatVND(500)).toBe("500 VND");
+    });
+  });
+
+  describe("formatUSD", () => {
+    it("formats with comma separator and 2 decimals", () => {
+      expect(formatUSD(1234.5)).toBe("$1,234.50");
+    });
+    it("handles zero", () => {
+      expect(formatUSD(0)).toBe("$0.00");
+    });
+  });
+
+  describe("formatCrypto", () => {
+    it("strips trailing zeros", () => {
+      expect(formatCrypto(0.001)).toBe("0.001");
+      expect(formatCrypto(1.0)).toBe("1");
+    });
+    it("preserves up to 8 decimals", () => {
+      expect(formatCrypto(0.12345678)).toBe("0.12345678");
+    });
+  });
+
+  describe("formatStock", () => {
+    it("floors to integer", () => {
+      expect(formatStock(1.7)).toBe("1");
+      expect(formatStock(150)).toBe("150");
+    });
+  });
+
+  describe("formatCurrency", () => {
+    it("dispatches to VND formatter", () => {
+      expect(formatCurrency(5000, "VND")).toBe("5.000 VND");
+    });
+    it("dispatches to USD formatter", () => {
+      expect(formatCurrency(100.5, "USD")).toBe("$100.50");
+    });
+    it("falls back for unknown currency", () => {
+      expect(formatCurrency(42, "EUR")).toBe("42 EUR");
+    });
+  });
+
+  describe("formatPnL", () => {
+    it("shows positive P&L", () => {
+      expect(formatPnL(12000000, 10000000)).toBe("+2.000.000 VND (+20.00%)");
+    });
+    it("shows negative P&L", () => {
+      expect(formatPnL(8000000, 10000000)).toBe("-2.000.000 VND (-20.00%)");
+    });
+    it("handles zero investment", () => {
+      expect(formatPnL(0, 0)).toBe("+0 VND (+0.00%)");
+    });
+  });
+});