类型重构：Chain Adapter 类型 → Amount #68

// src/services/chain-adapter/types.ts
export interface Balance {
  amount: Amount  // 替代 raw + formatted
  symbol: string
}

export interface Fee {
  amount: Amount  // bigint → Amount
  estimatedTime: number
}

export interface Transaction {
  amount: Amount  // bigint → Amount
  fee: Amount
  // ...
}