fix: reject malformed invoice amounts before creating payments

[zhangzhenggit]

parseAmount currently removes every $, comma and whitespace character before validating. This turns malformed input such as 1,2, 1$2 and 1 2 into a valid $12.00 invoice, which could charge a different amount from what the sender entered.

Validate the input first: allow a leading dollar sign, plain digits or properly grouped thousands, and up to two decimal places. Strip only the accepted formatting after validation. Add regression coverage for malformed separators, symbols and embedded spaces while preserving valid $1,200.50 and surrounding whitespace.

Validation: pnpm run build && node --test test/billing.test.ts (9 passing).