feat: add datetime picker for start field in TaskDialog

[Hell1213]

Added datetime picker for the start field in task edit dialog. Users can now select both date and time when editing the start field of existing tasks.

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
When users select only a date, it saves as YYYY-MM-DD format. When they select date + time, it saves as ISO format. No random time is added when users only pick a date.

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[Hell1213]

Replaced the basic date picker with a datetime picker for the start field in task editing. Now users can pick both date and time instead of just date, and it handles the formats properly on both frontend and backend.

[its-me-abhishek]

this should be just start

[Hell1213]

thanks , will resolve this immediately .

[its-me-abhishek]

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