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

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

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

@@ -0,0 +1,16 @@
+const MAX_DIGITS = 6;
+
+export function keypadReduce(current: string, key: string): string {
+  if (key === "back") return current.slice(0, -1);
+  if (!/^\d$/.test(key)) return current;
+  if (current === "0") return key === "0" ? "0" : key;  // replace leading zero
+  if (current === "" && key === "0") return "";          // no leading zero from empty
+  if (current.length >= MAX_DIGITS) return current;
+  return current + key;
+}
+
+export function withCommission(base: number, rate: number): { commission: number; total: number } {
+  const commission = Math.round(base * rate * 100) / 100;
+  const total = Math.round((base + commission) * 100) / 100;
+  return { commission, total };
+}