- Admin default-theme editor: "Theme importieren" button loads a .theme.json
into the editor (colours only, live preview) so an imported theme can be set
as the instance default (still confirmed via "Standard speichern").
- Branding: drop the "Standard" placeholder; show pixel-dimension hints instead
(logo shown at max 150×40, favicon 128×128). Logo preview box now mirrors its
real topbar footprint.
Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>