fix: show error when description is empty

[mulikruchi07]

Description

Fixes missing validation when editing a task description.
When the user presses Submit on an empty description, the dialog now remains open and shows an inline error message.

Fixes #513

Screenshots

Checklist

• Tests have been added or updated to cover the changes
• Documentation has been updated to reflect the changes
• Code follows the established coding style guidelines
• All tests are passing

[Copilot]

Pull request overview

This PR adds validation for empty task descriptions when editing through the description dialog. The implementation extracts the inline dialog code into a new _DescriptionEditDialog StatefulWidget to enable state management for displaying validation errors.

Key Changes:

• Converted the inline dialog builder to a separate _DescriptionEditDialog StatefulWidget
• Added validation to prevent empty descriptions with an inline error message
• Replaced TextField with TextFormField wrapped in a Form for error display

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[Copilot]

The _formKey is declared but never used. Consider either:

1. Using it with _formKey.currentState?.validate() to validate the form, or
2. Removing it if manual validation via errorText is preferred.

Currently, the code manually validates by checking controller.text.trim().isEmpty and setting errorText, which makes the Form wrapper unnecessary.