fix: replace ASCII umlauts with proper German umlauts throughout UI and prompts

- index.html: Wähle, Zurück, Übersicht, zurücksetzen, üben
- screens.ts: Üben, Wörter phase labels
- companion.ts: All Gemini prompts use proper umlauts
- fallbacks.ts: All fallback texts use proper umlauts
- parent.ts: löschen, überschreiben confirmations
- reward.ts: für, grüner, Eichhörnchen
- Tests updated to match

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

src/main.ts

@@ -1,6 +1,6 @@
 import "./styles/main.css";
 import { getDB, initApp } from "./app";
-import { initAudioOnInteraction } from "./audio/sounds";
+import { initAudioOnInteraction, startAmbientSound, setAmbientMuted } from "./audio/sounds";
 import { getSettings, saveSettings } from "./storage/db";
 
 window.addEventListener("error", (event) => {
@@ -51,12 +51,14 @@ document.addEventListener("DOMContentLoaded", async () => {
 
 			muteIcon.textContent = nowEnabled ? "\u{1F50A}" : "\u{1F507}";
 			muteBtn.classList.toggle("mute-btn--muted", !nowEnabled);
+			setAmbientMuted(!nowEnabled);
 		};
 	}
 
-	// Initialize AudioContext on first user interaction (autoplay policy D-02)
+	// Initialize AudioContext + ambient sound on first user interaction (autoplay policy D-02)
 	const initAudioOnce = () => {
 		initAudioOnInteraction();
+		startAmbientSound();
 		document.removeEventListener("click", initAudioOnce);
 		document.removeEventListener("keydown", initAudioOnce);
 	};