Add server-side native German TTS endpoint with MP3 disk caching

app.js

@@ -287,21 +287,51 @@ class WortKraftApp {
     if (tooltip) tooltip.classList.remove("visible");
   }
 
-  // === AUDIO PRONUNCIATION (Web Speech API de-DE) ===
-  speak(text, rate = 0.92) {
+  // === AUDIO PRONUNCIATION (Server Native German Neural TTS + Browser Fallback) ===
+  speak(text, rate = 0.96) {
+    if (!text) return;
+
+    // Stop any currently playing audio
+    if (this.currentAudio) {
+      try {
+        this.currentAudio.pause();
+        this.currentAudio.currentTime = 0;
+      } catch (_e) {}
+      this.currentAudio = null;
+    }
+    if ("speechSynthesis" in window) {
+      window.speechSynthesis.cancel();
+    }
+
+    // Primary: Stream authentic native German (de-DE) audio from backend /api/tts
+    if (window.location.protocol.startsWith("http")) {
+      const audio = new Audio(`/api/tts?text=${encodeURIComponent(text)}`);
+      audio.playbackRate = rate;
+      if ("preservesPitch" in audio) audio.preservesPitch = true;
+      this.currentAudio = audio;
+      audio.play().catch(err => {
+        console.warn("Server TTS playback error, falling back to browser TTS:", err);
+        this.speakBrowserFallback(text, rate);
+      });
+      return;
+    }
+
+    this.speakBrowserFallback(text, rate);
+  }
+
+  speakBrowserFallback(text, rate = 0.92) {
     if (!("speechSynthesis" in window)) {
       this.showToast("Audio wird von diesem Browser leider nicht unterstützt.");
       return;
     }
-    window.speechSynthesis.cancel();
     const utterance = new SpeechSynthesisUtterance(text);
     utterance.lang = "de-DE";
     utterance.rate = rate;
 
     const voices = window.speechSynthesis.getVoices();
     const germanVoice =
-      voices.find(v => v.lang.startsWith("de") && (v.name.includes("Natural") || v.name.includes("Google") || v.name.includes("Anna") || v.name.includes("Katja"))) ||
-      voices.find(v => v.lang.startsWith("de"));
+      voices.find(v => v.lang.toLowerCase().startsWith("de") && (v.name.includes("Natural") || v.name.includes("Google") || v.name.includes("Anna") || v.name.includes("Katja") || v.name.includes("Hedda") || v.name.includes("Stefan"))) ||
+      voices.find(v => v.lang.toLowerCase().startsWith("de"));
     if (germanVoice) {
       utterance.voice = germanVoice;
     }