fix(cli)(#36): invoice pay --amount accepts human units

[vrogojin]

Summary

• Closes invoice pay --amount: accept human units (toSmallestUnit conversion) for cross-command consistency #36 — the cross-command UX inconsistency PR fix(cli): bump SDK pin past #397 + treat invoice amounts as human units #35 left behind.
• sphere invoice pay <id> --amount 50 now means 50 whole UCT (or whatever the invoice's coin is), matching payments send 50 UCT and invoice create --asset 50 UCT.
• Picked Option A from the issue (single-token --amount, coin inferred from the invoice). Option B (--amount <value> <coin>) was rejected because forcing the user to re-type the coin the invoice already pins adds friction without information.

Implementation

invoice-pay resolves the invoice via sphere.accounting!.getInvoice(invoiceId), reads terms.targets[targetIndex].assets[0], and:

• fails fast on missing invoice / out-of-range target / NFT-asset target;
• looks up decimals through resolveCoin(symbol);
• pipes the raw value through toSmallestUnit(rawAmount, decimals);
• rejects zero / non-positive with failWithHelp.

This mirrors exactly the conversion path PR #35 added for invoice-create / invoice-return.

Help block updates

• description mentions the partial / over-payment use case
• --amount flag doc spells out human units and gives a fractional example
• examples use sphere invoice pay … form with human-readable amounts (0.5 UCT, 25 UCT)
• new notes line reminds operators that NFT targets reject --amount

sphere payments send @bob 50 UCT             → 50 whole UCT      ✓ human units
sphere invoice create --asset 50 UCT          → 50 whole UCT      ✓ human units (PR #35)
sphere invoice return … --asset 50 UCT        → 50 whole UCT      ✓ human units (PR #35)
sphere invoice pay <id> --amount 50           → 50 whole UCT      ✓ NOW human units

Out of scope

• --asset-index selector (target.assets[N] beyond N=0) — issue's "Out of scope" section, parked.
• SDK-level units semantics — PayInvoiceParams.amount stays a smallest-unit string. All conversion happens in the CLI.

Test plan

• npx tsc --noEmit — clean
• npx vitest run — 126/126 pass (legacy-cli-ux static-guard tests still green)
• node bin/sphere.mjs invoice pay --help — help block renders with new description / flag / examples / notes
• npx tsup — ESM + CJS + DTS build clean
• (optional follow-up) Extend manual-test-accounting-roundtrip.sh with a partial-payment leg exercising --amount — flagged in the issue as a nice-to-have to guard the new path

Closes #36.