Get all asset balances for the authenticated user.

Returns unlocked, locked, and in-flight amounts for each asset.

The unlocked value is the effectively available balance: raw
unlocked minus the total amount of any pending or ready
withdrawal requests for the same asset, clamped at zero.

fee_balances lists the user's prepaid trading-fee balances
(previously returned by GET /api/fees): each entry is a fee asset
with its available and in-flight amounts.