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/app.ts

@@ -0,0 +1,32 @@
+import type { ScreenName } from "./types";
+import { openDB, getProgress } from "./storage/db";
+import { initWelcomeScreen } from "./ui/screens";
+
+let db: IDBDatabase;
+
+export function showScreen(name: ScreenName): void {
+	document.querySelectorAll(".screen").forEach((el) => {
+		el.classList.remove("screen--active");
+	});
+	const target = document.getElementById(`screen-${name}`);
+	if (target) {
+		target.classList.add("screen--active");
+	}
+}
+
+export function getDB(): IDBDatabase {
+	return db;
+}
+
+export async function initApp(): Promise<void> {
+	db = await openDB();
+	const progress = await getProgress(db);
+	if (progress && progress.selectedCharacter) {
+		// Returning user — skip welcome, go to forest
+		showScreen("forest");
+	} else {
+		// First time — show welcome
+		initWelcomeScreen(db, showScreen);
+		showScreen("welcome");
+	}
+}