fix: resolve dark mode toggle functionality in navbar

[Satyam2307]

Description

This PR fixes the dark mode toggle button in the navbar that was not switching the application theme.

Changes Made

• Fixed theme toggle handler to correctly update theme state.
• Ensured ThemeProvider wraps the application.
• Applied theme changes to the document using data attributes/classes.
• Added theme persistence using localStorage.
• Verified dark theme styles are correctly applied across the application.
• Ensured the navbar toggle updates the UI immediately.

Result

• Clicking the moon icon now switches between light and dark themes.
• Theme changes are reflected across the application.
• User preference persists after page refresh.

fixes #839

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