Add Dark Mode / Light Mode Support to Frontend

[sasidaran-99]

Description

The frontend currently supports a single theme. Adding a dark/light mode toggle would improve accessibility and user experience, especially for users working on security reports and scan results for extended periods.

Proposed Changes

• Add a theme toggle button in the header/navbar.
• Support both dark and light themes across all major pages.
• Persist the selected theme using localStorage.
• Ensure findings, reports, forms, and navigation components adapt correctly to the selected theme.

Acceptance Criteria

• Users can switch between dark and light modes.
• Theme preference persists after page refresh.
• All major UI components render correctly in both themes.
• No accessibility or contrast issues are introduced.

[sasidaran-99]

@ionfwsrijan I'd like to work on this enhancement if it's approved. Please assign it to me if appropriate.