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..."

index.html

@@ -183,6 +183,14 @@
 			</div>
 		</section>
 
+		<div id="error-overlay" class="error-overlay" style="display:none">
+			<div class="error-overlay__content">
+				<p class="error-overlay__icon">&#x1F333;</p>
+				<p class="error-overlay__message">Oh, der Wald braucht kurz eine Pause...</p>
+				<button class="error-overlay__btn" onclick="location.reload()">Nochmal versuchen</button>
+			</div>
+		</div>
+
 		<script type="module" src="/src/main.ts"></script>
 	</body>
 </html>