Allow gifting the whole balance (no-change sweep) via Max

A gift PSBT commits a sender change output (>= dust), so the entered amount
always needed gift + 294 — making it impossible to gift the entire balance.
Add a Max toggle on the gift form that builds a no-change sweep: all inputs
SIGHASH_NONE, no change output, so the recipient gets the whole balance minus
their claim fee. previewGift/buildClaimTx now treat a zero-output PSBT as
zero change. A specific-amount gift that can't keep a dust change now points
the user to Max instead of failing with a confusing 'not enough funds'.

Co-Authored-By: Claude Opus 4.8 (1M context) <[email protected]>

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