feat: add datetime picker for due field in TaskDialog

[Hell1213]

Add DateTimePicker for due field in TaskDialog to allow users to select both date and time instead of just date.

Contributes: #326

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
This follows the same pattern as start field PR. Users can now pick specific times for due dates, not just dates.

[github-actions]

Thank you for opening this PR!

Before a maintainer takes a look, it would be really helpful if you could walk through your changes using GitHub's review tools.

Please take a moment to:

• Check the "Files changed" tab
• Leave comments on any lines for functions, comments, etc. that are important, non-obvious, or may need attention
• Clarify decisions you made or areas you might be unsure about and/or any future updates being considered.
• Finally, submit all the comments!

More information on how to conduct a self review:
https://docs.github.com/en/pull-requests/collaborating-with-pull-requests/reviewing-changes-in-pull-requests/reviewing-proposed-changes-in-a-pull-request

This helps make the review process smoother and gives us a clearer understanding of your thought process.

Once you've added your self-review, we'll continue from our side. Thank you!

[Hell1213]

Replaced DatePicker with DateTimePicker for the duefield in TaskDialog, updated backend to handle ISO datetime format, and fixed tests

[its-me-abhishek]

Please follow the commit conventions as set in CONTRIBUTING.MD while naming PRs and commits