feat(send): max amount button on the amount screen

[islandbitcoin]

Summary

Implements the approved design for a MAX button on Lightning sends: a compact chip in the amount screen's balance header, immediately right of the balance value.

Fixes #512

UX

• Chip states: outlined green (available) → solid green while the entered amount equals the computed max → back to outlined the moment the user edits the amount. Greyed out and non-interactive when the balance is zero. All colors come from the theme (colors.primary) — no hardcoded hex.
• Info row: while the chip is solid, a note under the amount explains the computation (fee reserved / no fee / recipient cap). An error message always takes precedence over the note.
• Currency toggle: toggling display currency keeps the same underlying amount, so the chip stays solid.

Max rules

Destination	Max	Note shown
Intraledger / Flash-to-Flash	Full wallet balance, no API call	"Full balance — no fee between Flash accounts"
External invoice / LNURL (BTC wallet)	Balance − fee estimate via fetchBreezFee (reuses the resolved payRequest from #680, no second LNURL round-trip)	"~$0.26 reserved for the network fee — final fee shown on confirm"
External invoice (USD wallet)	Balance − fee via the existing fee probes (useIbexFee), probed at the full balance	same as above
LNURL destinations	Clamped to min(computed max, maxSendable) — reuses the LUD-06 bounds validation shipped in #680; applies to BTC-wallet intraledger sends too since they settle via LNURL-pay	"Capped at … — the most this recipient can receive per payment"
Fee estimate fails / times out	Fills the full balance anyway; the existing pre-validation from #680 surfaces the typed fee error. The tap never blocks and never spinner-traps.	—

Architecture

amount-input-screen stays a generic component with zero payment knowledge. It gains an optional maxAmountButton?: { disabled?: boolean; compute: () => Promise<{ amount, note? } | null> } prop — when absent (all other usages of the amount screen), no chip renders and nothing changes.

The computation lives in the send flow:

• app/screens/send-bitcoin-screen/max-send-amount.ts — pure, dependency-free max rules (computeMaxSendAmount), unit-testable under plain jest (same pattern as fee-errors.ts from fix(send): surface real BTC fee errors and validate LNURL receive limits #680).
• send-bitcoin-details-screen.tsx builds the config from the payment detail (intraledger vs external vs LNURL, balances, resolved receiverPayRequest/receiverLimits) and threads it: DetailAmountNote → AmountInput → AmountInputModal → AmountInputScreen → AmountInputScreenUI.

The filled amount is floored in the primary display currency so it never converts back to more than the computed max in wallet units.

i18n

Four new AmountInputScreen keys in app/i18n/en/index.ts (max, maxNoteIntraledger, maxNoteFeeReserved, maxNoteRecipientCap). English placeholders appended to all 23 locale files under raw-i18n/translations/ for the next translation batch; generated i18n-types.ts and raw-i18n/source/en.json committed. No literal $ before interpolations — currency symbols arrive in the interpolated values.

Tests

• __tests__/utils/max-send-amount.spec.ts — 12 unit cases: intraledger = balance; external = balance − fee; LNURL clamp (binding, non-binding, on intraledger, on the fallback); fee null / throw / timeout fallbacks; zero balance; fee ≥ balance; negative/NaN fee.
• __tests__/components/amount-input-screen-max-chip.spec.tsx — 6 component cases in the house style of the fix(send): surface real BTC fee errors and validate LNURL receive limits #680 DetailAmountNote tests: no chip without the prop; outlined → tap fills the display-currency amount, goes solid, shows the note; editing returns it to outlined and hides the note; solid across currency toggle; greyed + inert at zero balance.

Checks run locally: yarn test 57 suites / 332 tests green, yarn tsc:check clean, eslint clean on changed lines, yarn check:translation-drift green, yarn update-translations idempotent (generated files committed).

🤖 Generated with Claude Code

https://claude.ai/code/session_015fAxSGEsL2LsMx1uCjAzHH

[islandbitcoin]

Device-testing follow-up: MAX on a fractional-cent USD balance ($1.099346) produced a $1.10 invoice and an IBEX insufficient-balance rejection. Fixed in the latest commit — the max computation now floors balance, fee-adjusted result, and recipient cap to whole minor units (sats unaffected), with regression tests pinning the exact repro numbers. Full suite 360/360 green.