fix : replaced alert with inline error in TaskPreview.jsx duration validation

[tmdeveloper007]

Summary of What Has Been Done

Replaced the browser alert() call in the TaskPreview component duration modal with a state-based inline error message for better UX.

Changes Made

• Added durationError state to the TaskPreview component
• Replaced alert() with setDurationError() when validation fails
• Cleared error on input change and modal close
• Added inline error display below the duration input

Impact it Made

• Eliminates jarring browser dialog
• Works properly on mobile
• Consistent with the app's UX patterns

Note: Please assign this PR to the tmdeveloper007 account.