Decodes file from GB-18030 to UTF-8

Removes first 4 lines

Remove trailing spaces in all cells

Replaces the following headers:

交易创建时间 → date 交易对方 → payee 商品名称 → notes 金额（元） → amount

Converts amount to number format

If 资金状态 column is not “已收入”, then amount is multiplied by -1

Converts date column to actual date format

Removes last 7 rows without data

Returns the following columns: date, payee, notes, amount