Add dark/light theme toggle to Settings

[Awaiswilll]

Description

Add a theme toggle in the Settings → General tab that allows users to switch between dark mode, light mode, and system theme.

What Needs to Be Done

• Add theme state management (dark/light/system)
• Create theme toggle UI component in Settings General tab
• Add CSS variables for light theme
• Persist theme preference in localStorage
• Apply theme on page load

Acceptance Criteria

• Theme toggle works in Settings → General
• Theme persists across page reloads
• System theme detection works
• All components look good in both themes

Relevant Files

• src/views/Settings.tsx — GeneralTab component
• src/styles/global.css — CSS variables
• src/App.tsx — Theme provider wrapper

Resources

• Contributing Guide
• Code Style