Add invoice payment currency converter (multi-asset)

[Kingsman-99]

Label: complexity: high
Points: 200

Description

For UIs displaying invoice amounts in a payer's preferred fiat/asset reference, this issue adds a converter using the contract's existing price-oracle pattern (mirroring price_oracle/get_price from the contract side) at the SDK level for arbitrary display conversions, not just payment-affecting ones.

Technical Context

New src/currencyConverter.ts. convertAmount(amount, fromToken, toDisplayCurrency, oracleAddress) calls the same oracle contract interface (get_price) the contract uses, but purely for display — does not affect payment amounts.

Acceptance Criteria

• convertAmount(...) returns a converted bigint/display string with no payment-side effects
• Caches oracle price lookups for priceCacheTtlMs (default 10s) to avoid spamming the oracle contract
• Clearly documented as display-only (not used for pay() calculations)
• Tests: conversion math correctness; cache hit avoids second oracle call

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