refactor: deprecate Amount type and unify to BalanceQuantity
- Made Amount a type alias to BalanceQuantity with deprecation notice - Updated asAmount() to delegate to asBalanceQuantity() - Replaced all Amount type annotations with BalanceQuantity - Replaced all Branded.asAmount() calls with Branded.asBalanceQuantity() - Removed all unnecessary 'as unknown as' type conversions - Updated API adapters, transaction builders, tests, and mocks - Maintained backward compatibility via deprecated Amount type alias This unifies the codebase to use BalanceQuantity consistently, eliminating confusion and unnecessary type conversions while maintaining compatibility.