Add dark mode toggle with system preference detection

Is your feature request related to a problem? Please describe. Currently the app only supports light mode, which can be harsh on the eyes in low-light environments and doesn't respect user OS-level theme preferences.

Describe the solution you'd like Add a dark mode toggle in the settings/preferences menu that allows users to:

Manually switch between light and dark themes Select "System" to automatically match the OS-level prefers-color-scheme setting Persist the user's choice across sessions (localStorage or user profile)

Describe alternatives you've considered Browser extensions that force dark mode via CSS injection — works but often breaks layouts and isn't a native experience Relying solely on OS-level dark mode without an in-app override — less flexible for users who want a different in-app preference than their system default

Additional context Would improve accessibility and user comfort, especially for users who work at night or have light sensitivity. Many similar tools (VS Code, GitHub itself, Slack) support this pattern already, so there's a good UX precedent to follow.