feat(01-02): screen management and companion definitions

- Create companion/characters.ts with 4 companion definitions (fee, einhorn, fuchs, eule)
- Create app.ts with showScreen, getDB, initApp (routes to welcome or forest)
- Update main.ts to call initApp on DOMContentLoaded
- Add stub ui/screens.ts for initWelcomeScreen

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

src/main.ts

@@ -1,3 +1,4 @@
 import "./styles/main.css";
+import { initApp } from "./app";
 
-console.log("Zauberwald loaded");
+document.addEventListener("DOMContentLoaded", initApp);