hyperbook: follow the page's dark-mode-toggle

A hyperbook carries a <dark-mode-toggle> its reader can flip, and an IDE
embedded in the page should be in the same colours as the text around it
rather than a dark rectangle in a light page.

With no `theme` in the config, the theme now comes from that element -
its `mode` property, falling back to the attribute for the moment before
the custom element has been upgraded - and follows it live through the
component's `colorschemechange` event, so flipping the toggle re-themes
the IDE without a reload. It re-reads once whenDefined() resolves as
well, so a toggle whose script loads after the IDE still gets honoured.

An explicitly configured theme still wins, and a page with no toggle
says nothing about its colours, so the IDE stays dark there.

Merge branch 'martin-pabst:master' into master

use new structure

use hyperbook dexie db

add lock file