feat: add pivot table CSV and JSON export

[Swarnabha753]

Summary

• Add CSV and JSON export options to the Pivot Table view.
• Export the pivot table using its currently configured rows, columns, and aggregation.
• Reuse the existing CSV/JSON export utilities.
• Add component tests covering CSV and JSON exports.

Testing

• bun test tests/components/PivotTable.test.tsx — 38 passed, 0 failed
• bun run typecheck — passed
• git diff --check — passed
• bun run test — 10,779 passed, 506 failed, 6 errors (existing unrelated repository test failures)

Closes: #749

[Swarnabha753]

Implemented the Pivot Table export functionality with both CSV and JSON formats, following the existing export pattern.

Also added tests covering both export formats.

Validation:

• PivotTable tests: 38 passed, 0 failed
• Typecheck: passed
• git diff --check: passed

Ready for review. Thanks!

[Swarnabha753]

The Unit & Integration Tests check is failing in the existing DatabaseDocs smoke tests, unrelated to this PR:

• DatabaseDocs > Export MD button creates and clicks download link
• DatabaseDocs > Export MD includes AI Analysis section when AI docs exist

Both fail because createObjectURLMock receives 0 calls. The PivotTable tests pass locally, and this PR does not modify DatabaseDocs or its tests.

Lint, Typecheck and Build and Channel E2E checks are passing.