test: cover preferred export format hook

[saurabhhhcodes]

Summary

• add direct unit coverage for usePreferredExportFormat
• assert empty default state, stored preference restoration, and savePreference persistence
• keep the coverage isolated from page-level report components

Fixes #559

Validation

• npm test -- --run testing/unit/hooks/usePreferredExportFormat.test.ts
• git diff --check

[utksh1]

Please rebase and narrow this PR to the preferred-export-format hook coverage only. It currently carries the same CI baseline repair that is already handled by #629/#635, so merging it as-is would duplicate unrelated backend/workflow/test-baseline changes.