Bring the README up to date and add a German translation

The feature list still described six colors and a copy/share/save toolbar,
both of which the preceding commits had already outgrown: white is a seventh
color with its own dark-appearance variant, the toolbar also pins and
recolors, and launching at login was missing entirely.

LIESMICH.md is the German counterpart; the two link to each other.

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