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.

No files, tests, or entry points are named. Start by locating the settings/preferences UI and existing theme styles, then review how prefers-color-scheme and localStorage could fit the app; done means users can choose Light, Dark, or System and the choice persists across sessions.