fix : replaced alert call with toast notification in DashboardTasks

[tmdeveloper007]

Summary of What Has Been Done

Replaced the browser alert() call in the DashboardTasks.jsx component with a toast notification for duration validation errors.

Changes Made

1. Added a local Toast component (matching the pattern used in Tasks.jsx) that renders at the bottom-right of the screen.
2. Added toast state and a showToast helper function with auto-dismiss after 3000ms.
3. Replaced alert("Please enter a valid duration in minutes") with showToast("Please enter a valid duration in minutes", "error").
4. Added useEffect cleanup to clear the toast timer on unmount.
5. Added dark-mode-aware styling to the duration modal (dark background, light text, proper border colors).

Impact it Made

• Users see a polished, non-blocking toast notification instead of a jarring browser alert.
• Duration modal is now dark-mode compatible.
• Consistent with the toast-based UX pattern used elsewhere in the application.

Note: please assign this PR to the tmdeveloper007 account.