Add data import from CSV/Excel

[ravindu2012]

Description

Allow users to import data from CSV or Excel files to quickly set up their books.

Import Types

• Chart of Accounts
• Customers
• Vendors
• Items/Products
• Opening balances

Features

• File picker dialog
• Column mapping UI
• Validation preview before import
• Error reporting for invalid rows

Suggested Libraries

• `CsvHelper` for CSV parsing
• `EPPlus` or `ClosedXML` for Excel