Fix Theme-Specific Icon Display in TailNG UI #85

The TailNG UI theme control currently displays the same icon regardless of the active theme. The icon should change based on whether the application is in light mode or dark mode.

Issues

• In light mode, the theme control should display the moon icon, indicating that the user can switch to dark mode.
• In dark mode, the theme control should display the sun icon, indicating that the user can switch to light mode.
• The current implementation does not correctly change the displayed icon when the theme changes.

Expected Behavior

• Light mode → Moon icon
• Dark mode → Sun icon
• The displayed icon should update immediately when switching themes.
• The icon change should use the existing TailNG Icon component consistently.
• No changes to icon color behavior are required.