Show the fee the transaction pays, not the one the response claims

[droplister]

PRIORITIES.md item 1.

The problem

checkTransactionFee already recomputes the miner fee from the transaction's own inputs and outputs, resolving input values from the chain rather than from the response (ADR-019 layer 3). That number was used to accept or reject the transaction — and then discarded. The review screen rendered result.btc_fee: the composer's own assertion about itself.

The bound that is enforced is deliberately loose, because a legitimate composer needs room:

max(10_000, rate × vsize × 10)

transaction	honest fee	max that still passes	overpay
250 vB @ 2 sat/vB	500	10,000	9,500 (20×)
400 vB @ 10 sat/vB	4,000	40,000	36,000 (10×)

So a response could pass verification while reporting a smaller fee than the transaction actually pays, and the user would sign against the number on screen. Value goes to miners rather than an attacker, so this is griefing rather than theft — but the displayed number was simply not the truth.

The change

The composer stores the computed fee in place of the reported one. Because all 28 review screens render result.btc_fee, that corrects every one of them at a single point rather than touching each screen.

• A response that stated a different fee also produces a warning — contradicting a stated value is worth surfacing to the user.
• A response that stated no fee is corrected silently. Filling in a value nobody claimed is not a discrepancy, and warning about it would be noise. (This distinction is why two composer tests initially failed: their fixtures omit btc_fee entirely.)

The test that was already broken

The existing should compose transaction test turned out to be a live example of the bug. Its fixture asserts btc_fee: 5000, while its stubbed input value (100,000) minus its single 95,160-sat output leaves 4,840. The fixture had been internally inconsistent all along and nothing checked — the defect in miniature. The test now asserts the corrected value and the warning.

Verification

• tsc --noEmit clean.
• 2524 unit tests across contexts/components/pages/blockchain.
• E2E locally: compose/send/index.spec.ts 20/20, which includes "review page shows correct transaction details".

https://claude.ai/code/session_01CcjnCrgosSeshymXLBxdGj