fix: set UTF-8 client charset for SQL Server connections

[datlechin]

Summary

• Set DBSETCHARSET to UTF-8 on the FreeTDS login record so Unicode data (Thai, CJK, Arabic, emoji, etc.) in nvarchar/nchar/ntext columns is delivered as UTF-8 instead of being mangled through the server's default single-byte codepage
• Updated columnValueAsString to try UTF-8 decoding first for nvarchar types (with UTF-16LE fallback), since FreeTDS converts wire data to the client charset but may retain the original type token

Without this fix, any characters outside the server's default codepage (e.g., cp1252) appear as ? in TablePro. Reported by a user with Thai text in nvarchar columns on Azure SQL Server.

Test plan

• Connect to a SQL Server instance with an nvarchar column containing Thai text (e.g., N'สวัสดี') — verify it displays correctly
• Verify Latin/ASCII text in varchar and nvarchar columns still displays correctly
• Test with other Unicode scripts (CJK, Arabic, emoji) in nvarchar columns
• Verify ntext columns also render Unicode correctly