fix: convert cash send amounts at API boundary

[forge0x]

Summary

• fixes BUG-004 by treating backend cash wallet balances, fees, and payment mutation inputs as USD cents at GraphQL boundaries
• keeps USDT wallet settlement amounts in micros inside the mobile send flow
• converts backend cash fee responses back into the active wallet unit before display/balance validation

UAT

• confirmed on MacMax physical-device UAT that SEND-01 and SEND-02 now pass
• verified send methods that share this path no longer throw the false $1.23/$1.24 (amount+fee) balance error for $0.05 sends

Tests

• yarn test __tests__/payment-details __tests__/payment-destination --runInBand
• yarn tsc:check
• yarn graphql-check
• yarn check:codegen
• git diff --check

Stacked against #621 (feat/fygaro).

[islandbitcoin]

⚠️ Heads-up: 3 pre-existing eslint errors in send-bitcoin-confirmation-screen.tsx

Not introduced by this PR — I confirmed they're present on the untouched branch head too, and they're unrelated to the cash/USDT unit fix, so I left them alone rather than expand the diff. Flagging because a full-file eslint run will fail on them (a diff-/changed-lines-only lint won't):

Line	Rule	Issue
52:14	@typescript-eslint/ban-types	Uses {} as a type (means "any non-nullish value" — likely wants object / Record<string, never>)
270:59	no-implicit-coercion	!!paymentError-style coercion — use Boolean(paymentError)
281:33	@typescript-eslint/no-unused-vars	colors defined but never used

✖ 5 problems (3 errors, 2 warnings)
  1 error and 0 warnings potentially fixable with the `--fix` option.

(#270 and #281 are auto-fixable via eslint --fix; the {} type at #52 needs a manual decision on the intended type.)

These would be a quick separate cleanup commit if you want the file to pass a whole-file lint — happy to take it on if useful, but kept it out of this PR to avoid unrelated churn.

[islandbitcoin]

Closed by lnflash/flash#413 commit lnflash/flash@2a9e2ae