Add dark mode toggle

[abby-ql]

Closes #382
Helium currently supports dark mode but users cannot manually override it per site.
This PR adds a manual color mode toggle to the Helium theme. Users can switch between Auto (follow prefers-color-scheme), Light, and Dark. The selected mode is stored locally in the browser using localStorage and persists across page navigations. If no selection is made, the default is Auto which follows the system/browser preference, and no personal or identifying data is stored. The toggle button is placed on top left navigation bar.
Testing: checked Auto does follow browser default, and Light/Dark mode switch is working.

Example ( using the protosearch doc)

[abby-ql]

The current implementation renders the toggle (and the initialization script) by default, which changes the generated html structure and causes several Helium html tests to fail due to expected output differences. I have checked functionally everything works as intended in the browser.

Would like to check whether enabling the toggle by default ( which would require updating a whole bunch of tests) is preferred, or if it should instead be opt in via Helium configuration flag, thank you!