fix : removed alert call from TaskPreview duration validation

[tmdeveloper007]

Summary of What Has Been Done

Replaced the blocking alert() call in frontend/src/components/Dashboard/TaskPreview.jsx with an inline error state:

• Added durationError state variable
• Replaced alert("Please enter a valid duration in minutes") with setDurationError(...)
• Added inline error display in the duration modal below the input field
• Cleared error on input change, modal cancel, and successful submission

Changes Made

• frontend/src/components/Dashboard/TaskPreview.jsx: alert removed, inline error feedback added

Impact it Made

• Removes blocking alert() dialog from the task completion flow
• Provides inline, non-intrusive error feedback instead of disruptive browser dialogs

Closes #1945.

Note: please assign this PR to the tmdeveloper007 account.