Fix Max button, slash scan requests (429s), cache history locally

Max button: fill the amount field with the computed maximum (n inputs, 1
output, current fee rate) so it gives visible feedback instead of just
blanking/disabling the field.

Rate limits: eliminate look-ahead (gap limit 1 — stop at the first unused
address, safe because we only ever expose one unused address at a time),
query one address at a time (API concurrency 1, 400ms spacing across two
backends), scan chains sequentially, and pause 500ms after each used address.
An empty scan is now ~2-3 requests instead of ~10+.

History cache: persist scanned state (addresses, utxos, txs, indices) in
localStorage keyed by sha256(seed) — the seed itself is never stored. Re-
importing the same seed in this browser shows the last-known balance/history
instantly while a fresh scan runs. Cleared by nothing automatically (public
chain data); different seeds get separate keys.

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

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