docs: update STATE.md with post-milestone work and update plugin author

- Document UI/UX modernization, email rewrite, and dev tooling in STATE.md
- Add post-milestone decisions table entries
- Update session continuity to 2026-02-13
- Update plugin author

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

umzugsliste.php

@@ -3,7 +3,7 @@
  * Plugin Name: Umzugsliste
  * Description: Email-basiertes Möbelauswahlsystem für Siegel Umzüge
  * Version: 1.0.0
- * Author: Siegel Umzüge
+ * Author: Viktor Miller
  * Text Domain: siegel-umzugsliste
  * Domain Path: /languages
  */