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/ui/screens.ts

@@ -0,0 +1,8 @@
+import type { ScreenName } from "../types";
+
+export function initWelcomeScreen(
+	_db: IDBDatabase,
+	_navigateTo: (screen: ScreenName) => void,
+): void {
+	// Implemented in Task 2
+}