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/styles/main.css

@@ -964,6 +964,45 @@ h3 {
 	}
 }
 
+/* Error overlay — kindgerechte Fehlermeldung */
+.error-overlay {
+	position: fixed;
+	inset: 0;
+	background: rgba(255, 248, 240, 0.95);
+	z-index: 9999;
+	display: flex;
+	align-items: center;
+	justify-content: center;
+}
+
+.error-overlay__content {
+	text-align: center;
+	padding: 2rem;
+}
+
+.error-overlay__icon {
+	font-size: 4rem;
+	margin-bottom: 1rem;
+}
+
+.error-overlay__message {
+	font-family: "Quicksand", sans-serif;
+	font-size: 1.25rem;
+	color: var(--text-dark);
+	margin-bottom: 1.5rem;
+}
+
+.error-overlay__btn {
+	font-family: "Quicksand", sans-serif;
+	font-size: 1rem;
+	padding: 0.75rem 2rem;
+	background: var(--accent-green);
+	color: white;
+	border: none;
+	border-radius: 12px;
+	cursor: pointer;
+}
+
 /* Accessibility: respect reduced motion preference */
 @media (prefers-reduced-motion: reduce) {
 	.falling-letter,