fix(convert): auto-correct the entry to the affordable max instead of prompting

[bmc08gt]

Problem

Amount entry is capped at the raw balance, but the fee comes out of that same balance:

• on top — the funding side is Dollars, which has no launchpad sale to skim, so the debit is entered × (1 + f)
• grossed up — every other currency, where the pool's sell fee is taken out of the sale, so the debit is entered / (1 - f)

Either way, entering the maximum always overruns the balance by exactly the fee — and never by more.

Both flows handled that badly:

• Get / Buy priced the number the user typed, landed on the receipt, and then popped an "Insufficient Balance After Fees" modal asking them to confirm dropping to the maximum. Accepting only corrected the receipt, so going back to amount entry still showed the old number.
• Convert out of Dollars had no modal at all — converting the full balance hard-failed at submit with "Insufficient Funds".

Fix

Trim the entry itself to what the balance can actually fund, before anything is priced. The correction band is only ever the fee percentage (checkFundingAmount/checkBalanceLimit already block anything above the raw balance), so there is nothing meaningful to put to the user.

New pure helper entryAffordableAfterFee inverts both conventions:

fee shape	debit	affordable max
on top	entered × (1 + f)	balance / (1 + f)
grossed up	entered / (1 - f)	balance × (1 - f)

The result is floored to the currency's smallest unit via a new Fiat.flooredToSmallestUnit(). Rounding it up would put the debit straight back over the balance — $10.11 at 1% corrects to $10.00, not $10.01, because $10.01 plus its own fee is $10.1101.

The correction writes through a new AmountEntryDelegate.setAmount (a replace-setter — prefill types on top of the existing entry), so the amount screen and the receipt show the same number when the user navigates back.

Buy now derives its fee shape once through buyFeeFor(token), feeding both the correction and the receipt's exchangeFee, so the two can't drift apart. The dead modal branch and its three strings are removed.

Tests

16 new tests, written test-first:

• FeeAffordableEntryTest (8) — entries with room left alone, zero-fee no-op, whole balance under each fee convention, the flooring case, idempotence, over-balance entries, and a JPY case confirming the correction is denominated in the balance's currency
• LaunchpadSellFeeTest (4) — the two new inverses round-trip back to the balance
• FiatTest (3) — flooring
• AmountEntryDelegateTest (1) — setAmount replaces rather than appends