Dark mode toggling mechanism

We should support a three‑state theme toggle that defaults to the system setting via the [prefers-color-scheme](https://developer.mozilla.org/en-US/docs/Web/CSS/Reference/At-rules/@media/prefers-color-scheme) feature.

Users must also be able to override this behavior using an in‑app toggle (e.g., light, dark, or system). When a manual override is selected, the preference should be persisted in `localStorage` and take precedence over the system setting until changed.