add: Allow empty start date to default to today in tmpo manual

[DylanDevelops]

Pull Request Checklist

• I have read and followed the contribution guidelines.
• My pull request targets the main branch of tmpo.
• I have tested these changes locally on my machine.

Closes #82

Description

This pull request updates the manual entry command to improve the user experience when entering the start date. The changes make the start date prompt clearer and allow users to quickly accept today's date by pressing enter.

User experience improvements for date entry:

• The start date prompt now displays today's date as the default and allows users to edit the value or accept the default by pressing enter. If the input is left blank, today's date is automatically used. [1] [2]

[jkrasko-cu]

Good quality of life change, DylanDevelops!