fix: honor DateInput defaultValue in single-date mode

[Henry369-0]

Summary

• honor an ISO defaultValue for DateInput in single-date mode
• keep the initialized value from being overwritten by the fallback range initialization
• add a Storybook example covering the single-date default value

Validation

• verified the branch against evidence-dev/evidence:main
• reviewed the GitHub compare diff for the two intended files
• local test suite was not run because the Windows Git HTTPS credential layer could not clone the repository

Closes #3312

[Henry369-0]

Closing this draft of the contribution while the contributor profile privacy settings are being reviewed. No maintainer action is required.