Export results to CSV #33

Description

Export the current result set to a CSV file.

Implementation

• Tauri save dialog for file path selection
• Large exports: sidecar writes CSV directly to disk (streaming, no memory pressure)
• Small exports (< 10K rows): frontend generates CSV from in-memory data

Acceptance Criteria

• "Export to CSV" button in results toolbar
• Tauri save dialog with .csv filter
• Handles all data types correctly (quoting, escaping)
• Large result sets exported via sidecar without memory issues
• Progress indicator for large exports
• UTF-8 encoding with BOM for Excel compatibility