feat(ui): add universal data export (CSV, JSON, Markdown, SQL dump) to ResultsTab

[ZhuchkaTriplesix]

Summary

Adds universal data export capabilities (DataExportService) and upgraded export menus to ResultsTab, supporting CSV, JSON, GitHub Flavored Markdown tables, and SQL INSERT dumps.

Closes #326
Part of Epic #322

Changes

• Created DataExportService (lib/shared/services/data_export_service.dart):
  • formatCsv(columns, rows)
  • formatJson(columns, rows, {asObjectArray = true})
  • formatMarkdownTable(columns, rows) (handles escaping pipes | and \n)
  • formatSqlInsertDump(tableName, columns, rows) (handles single-quote escaping and NULL tokens)
  • formatAsync to execute large grid formatting on background worker isolates (Isolate.run) to prevent UI stutter.
  • copyToClipboard & saveToFile integrating file_selector and Clipboard.setData.
• Upgraded ResultsTab (lib/features/main_screen/results_tab.dart):
  • Retained fast-access Copy as CSV and Copy as JSON buttons.
  • Replaced clutter with two clean dropdown action menus:
    • Copy formatted ▾ (CSV, JSON, Markdown Table, SQL INSERT Dump)
    • Save to file ▾ (CSV (.csv), JSON (.json), Markdown Table (.md), SQL INSERT Dump (.sql))

Verification

• Ran flutter test test/shared/data_export_service_test.dart (All unit tests passed verifying exact character/quote/pipe escaping).
• Ran flutter analyze across project (No issues found).
• Ran full regression test suite flutter test (All 811 tests passed).