Add dark mode toggle to settings page

[tomkis]

Summary

Add a dark mode toggle to the settings page. By default, the toggle should respect the user's OS-level color scheme preference (via the prefers-color-scheme media query), while allowing the user to manually override and lock the theme to light or dark mode.

Details

Type

feature

Priority

P2 (medium)

Acceptance Criteria

• A dark mode toggle is visible and accessible on the settings page
• On first load (no saved preference), the app respects the OS prefers-color-scheme setting
• The user can manually switch between light and dark mode using the toggle
• The user's manual preference is persisted (e.g., localStorage) and takes precedence over the OS preference on subsequent visits
• Toggling dark mode applies the theme change immediately without a page reload

Additional Context

This is a commonly expected accessibility and UX feature. Respecting the OS preference by default ensures a good out-of-the-box experience for users who have already configured their system preference.

---

Filed via file-issue skill