feat: add time to due date

[Vasist10]

Description

This PR adds explicit date + time selection support for time-based Taskwarrior fields, starting with due.

• Replaced DatePicker with a new DateTimePicker for the due field

• Clear separation between date-only and date + time input

• Time is included only when the user explicitly selects it

• Backend now handles incoming date/datetime values to Taskwarrior format

• Updated tests regarding the same

• Fixes: 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

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[its-me-abhishek]

This looks great overall, please look into these changes, will merge later once these get fixed