fix : added replaced hardcoded text-black in duration completion modals

[tmdeveloper007]

Summary of What Has Been Done

Replaced hardcoded text-black in the Complete Task duration modal with theme-aware classes for dark mode support in both DashboardTasks.jsx and TaskPreview.jsx.

Changes Made

• Replaced modal bg-white with dark mode variant
• Replaced text-black/90 with text-main
• Replaced text-black (body) with text-muted
• Replaced text-black (input) with dark mode variant
• Replaced text-black (cancel button) with dark mode variant
• Added dark:bg-black/40 to modal backdrop

Impact it Made

Complete Task modal is readable in dark mode.

Closes #1989

Note: Please assign this PR to the tmdeveloper007 account.