test: add coverage for presentational components

[Soengkit]

Summary

• Add Button tests for default/secondary/danger variants, disabled state, and prop/className forwarding.
• Add EmptyState and PageHeading tests for optional description/action slots being present and omitted.
• Extend Card coverage for omitted optional regions and div prop forwarding.

Closes #13

Validation

• npm run lint
• npm test -- --coverage
• NEXT_TELEMETRY_DISABLED=1 npm run build

Coverage note: Button, Card, EmptyState, and PageHeading report 100% statement/branch/function/line coverage.