fix: close the defects an adversarial audit found

An outside integrator and a code audit both concluded the merchant half was
a demo wearing a product's clothes. What they proved, and what changed:

- The watcher confirmed on ABSOLUTE balance, so an address that already held
  funds marked an invoice paid the moment it was created and the merchant
  shipped goods nobody paid for. Payment is now a delta from a baseline read
  at registration.
- A payment that landed before the deadline lost to an expiry evaluated
  after it, stranding the payer's funds. Payment now wins.
- The API had no auth and wildcard CORS, so any page the merchant visited
  could read the whole invoice ledger, inject invoices, and, via a
  caller-supplied webhook URL, make the watcher sign arbitrary bodies with
  the merchant's secret. Bearer token required, exact-origin CORS, and the
  webhook URL is server configuration only.
- Signatures covered the body alone and replayed forever. They now cover
  timestamp.body, expire in five minutes, and carry a deliveryId to dedupe
  on. An unset secret refuses to send rather than sending unsigned.
- Retry after a failed confirmation re-broadcast the payment. The sent hash
  is remembered, so a retry only waits.
- The hosted page interpolated URL params into innerHTML: any link could run
  script on the payment origin and rewrite the address being paid. Built
  from DOM nodes now, with the address and amount validated first.
- resolveToken assumed 18 decimals for unknown addresses, a 10^12 error on a
  6-decimal token. It refuses instead. Empty and multi-dot amounts parsed as
  0 and as truncated values; both throw.
- The receipt copied the invoice's network claim; it now reports the
  wallet's, and a network mismatch is refused before any prompt.
- e2e-live.mjs proved confirmation by watching a funded address and
  reporting paid. That was a false positive presented as evidence. It now
  asserts the opposite, plus signature replay rejection.

51 tests, up from 31: the HTTP surface had none at all. Docs corrected too,
including a Tier 1 install that never worked and privacy claims the code did
not support.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>