Add theme-adaptive screenshots to README and fix manifest icons

Replace old screenshots with new mobile/tablet variants in light and dark
mode. README now uses <picture> elements with prefers-color-scheme so
screenshots automatically match the viewer's GitHub theme. Split manifest
icon purpose field into separate "any" and "maskable" entries per PWA spec.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

.gitignore

@@ -33,3 +33,4 @@ dist/
 # Textdateien mit Tokens/Keys (Sicherheitsnetz)
 *.txt
 !public/robots.txt
+CLAUDE.md