feat(AddTaskDialog): add DateTimePicker for start field

[ShivaGupta-14]

Description

• Replace DatePicker with DateTimePicker for start field in AddTaskDialog

• Add DateTimePicker mock in AddTaskDialog.test.tsx

• Add comprehensive tests for DateTime fields (render, update, clear, submit)

• Support both date-only and datetime formats

• Contributes: Add additional time picker for time(and date) based fields in AddTaskDialog #325

Checklist

• Ran npx prettier --write . (for formatting)
• Ran gofmt -w . (for Go backend)
• Ran npm test (for JS/TS testing)
• Added unit tests, if applicable
• Verified all tests pass
• Updated documentation, if needed

Additional Notes

Screenshots

[github-actions]

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[ShivaGupta-14]

self review done, ready for review

[ShivaGupta-14]

Hi @its-me-abhishek , thanks for the approval!
Is there anything needed from my side for merging?

[ShivaGupta-14]

requested changes are done, happy to iterate based on your feedback

[ShivaGupta-14]

@its-me-abhishek could you please review this