feat(04-02): global error handler + child-friendly error overlay

- Add error overlay HTML element in index.html before script tag
- Add window error + unhandledrejection listeners in main.ts
- Add error overlay CSS styles (fixed, z-index 9999, tree icon, reload button)
- Error message: "Oh, der Wald braucht kurz eine Pause..."

src/main.ts

@@ -1,4 +1,16 @@
 import "./styles/main.css";
 import { initApp } from "./app";
 
+window.addEventListener("error", (event) => {
+	console.error("Global error:", event.error);
+	const overlay = document.getElementById("error-overlay");
+	if (overlay) overlay.style.display = "flex";
+});
+
+window.addEventListener("unhandledrejection", (event) => {
+	console.error("Unhandled rejection:", event.reason);
+	const overlay = document.getElementById("error-overlay");
+	if (overlay) overlay.style.display = "flex";
+});
+
 document.addEventListener("DOMContentLoaded", initApp);