fix : apply dark mode ring color and text to tag selection buttons in TaskFormModal

[tmdeveloper007]

Closes #1884

Summary of What Has Been Done

Added proper dark mode ring color and text color to the tag selection buttons in frontend/src/components/Task/TaskFormModal.jsx.

Changes Made

• Added ring-indigo-500 dark:ring-indigo-400 to selected tag buttons so the ring is visible in both light and dark mode
• Added text-slate-700 dark:text-slate-300 to unselected tag buttons for better contrast in dark mode

Impact it Made

• Tag selection ring is clearly visible in dark mode (previously used browser-default ring color)
• Unselected tag labels have proper contrast in dark mode, fixing low-visibility issue
• Consistent dark mode experience when creating or editing tasks

Note: Please assign this PR to the tmdeveloper007 account.