feat(driver-pwa): pure money/phone/amount helpers + tests

Co-Authored-By: claude-flow <ruv@ruv.net>

driver-pwa/frontend/src/lib/amount.test.ts

@@ -0,0 +1,19 @@
+import { describe, it, expect } from "vitest";
+import { keypadReduce, withCommission } from "./amount";
+
+describe("amount keypad", () => {
+  it("appends digits, ignores leading zero, caps length", () => {
+    expect(keypadReduce("0", "5")).toBe("5");
+    expect(keypadReduce("100", "0")).toBe("1000");
+    expect(keypadReduce("", "0")).toBe("");
+    expect(keypadReduce("999999", "9")).toBe("999999");
+  });
+  it("backspace removes last digit", () => {
+    expect(keypadReduce("105", "back")).toBe("10");
+    expect(keypadReduce("", "back")).toBe("");
+  });
+  it("withCommission computes total at given rate", () => {
+    expect(withCommission(1000, 0.052)).toEqual({ commission: 52, total: 1052 });
+    expect(withCommission(333, 0.052)).toEqual({ commission: 17.32, total: 350.32 });
+  });
+});