[Feature] Add dark mode support

[rohitthink]

Problem

The app currently only has a light theme. Many users prefer dark mode, especially when working late at night during tax season.

Proposed Solution

Add a dark mode toggle to the settings page that switches the app between light and dark themes.

Suggested approach

• Use Tailwind CSS dark: variant classes
• Add a theme toggle in the settings page and navbar
• Store preference in localStorage
• Respect system preference via prefers-color-scheme

Files to modify

• frontend/src/app/layout.tsx — add dark class support
• frontend/src/app/settings/page.tsx — add toggle
• Component files — add dark: Tailwind variants

This is a great first issue for someone familiar with Tailwind CSS and React.

[Cenaphie]

OK, I’ll implement dark mode