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/audio/sounds.ts

@@ -2,6 +2,8 @@ import { getDB } from "../app";
 import { getSettings } from "../storage/db";
 
 let audioCtx: AudioContext | null = null;
+let ambientAudio: HTMLAudioElement | null = null;
+let ambientStarted = false;
 
 /**
  * Creates or resumes AudioContext on first user interaction.
@@ -21,6 +23,39 @@ export function initAudioOnInteraction(): void {
 	}
 }
 
+/**
+ * Start ambient forest sounds (loops quietly in background).
+ * Called once on first user interaction alongside AudioContext init.
+ */
+export async function startAmbientSound(): Promise<void> {
+	if (ambientStarted) return;
+	ambientStarted = true;
+	try {
+		if (!(await isAudioEnabled())) return;
+		ambientAudio = new Audio("/forest-birds.mp3");
+		ambientAudio.loop = true;
+		ambientAudio.volume = 0.08;
+		ambientAudio.play().catch(() => {
+			// Autoplay blocked — will retry on next interaction
+			ambientStarted = false;
+		});
+	} catch {
+		ambientStarted = false;
+	}
+}
+
+/**
+ * Update ambient sound state based on mute toggle.
+ */
+export function setAmbientMuted(muted: boolean): void {
+	if (!ambientAudio) return;
+	if (muted) {
+		ambientAudio.pause();
+	} else {
+		ambientAudio.play().catch(() => {});
+	}
+}
+
 async function isAudioEnabled(): Promise<boolean> {
 	try {
 		const db = getDB();