Add Ctrl+Y to copy results to clipboard

[rohitgs28]

Summary

Copy the current result set to the system clipboard as tab-separated values.

Expected Behavior

• Ctrl+Y in the Results panel copies all visible rows as TSV to clipboard
• Status bar shows "Copied X rows to clipboard"
• Include column headers

Technical Notes

• The arboard crate is already in Cargo.toml
• Result data is in app.result_columns and app.result_rows
• Similar to the existing export_csv() function but outputs to clipboard instead of file
• Add keybinding in handle_key() Results panel match arm

This should be straightforward for someone familiar with Rust. Around 20 lines of code.