Component: Dark/light/system mode toggle

Currently, dark mode and light mode are controlled by listening for a user's system settings or applying a `.kelp-theme-dark` or `.kelp-theme-light` class.

Some users may prefer to provide a button or select menu that explicitly lets users choose between dark/light/system modes.

This web component would add controls that integrate into the existing dark/light mode system and script as an enhancement. User preferences to be stored in localStorage under the `kelp-theme` key.

Start by locating the existing dark/light mode component and script that respond to system settings and the `.kelp-theme-dark` or `.kelp-theme-light` classes. Trace how those controls can expose dark, light, and system choices, then verify that the selected preference is stored under the `kelp-theme` localStorage key and survives a reload.

Written by the indexing model from the issue text.