Follows Obsidian light/dark theme

fix: epub reader now follows Obsidian light/dark theme toggle after open

Previously isDarkMode was sampled once at mount with no subscription, so
switching the theme after an epub was already open left the rendition's
color/background overridden on the initial theme. Make isDarkMode
reactive via a MutationObserver on body's class, and re-apply the
rendition theme in an effect on change. Bump version to 1.0.4.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>