Copy results to clipboard

[GordonBeeming]

Description

Copy selected cells or entire result grid to clipboard in TSV format.

Acceptance Criteria

• Ctrl+C copies selected cells as tab-separated values
• Column headers included when copying entire columns
• Copies correctly paste into Excel/Google Sheets
• "Copy" button in results toolbar for entire result set
• "Copy with Headers" option
• NULL values copied as empty string