Add datetime picker for end field in TaskDialog

[Hell1213]

Added DateTimePicker component to the end field in TaskDialog, allowing users to select both date and time instead of date-only. Updated backend to handle ISO datetime format conversion and updated tests accordingly.

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 and due fields. Frontend uses DateTimePicker with hasTime parameter for format selection (ISO vs YYYY-MM-DD). Backend converts ISO format to Taskwarrior format.

[github-actions]

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[Hell1213]

done with review ,we are following the same pattern as start and due fields