Fix TTS preview for 'Server default' voice option

- Allow empty voice value to preview server default
- Display 'server default' in preview text
- Clears user preference (sets to null) when testing default

public/js/voicePreferences.js

@@ -159,13 +159,9 @@
     var ttsSelect = document.getElementById('tts-voice-select');
     var voice = ttsSelect ? ttsSelect.value : null;
 
-    if (!voice || voice === '') {
+    // Allow "Server default" (empty value) to preview
-      showToast('Please select a voice from the dropdown first', 'info');
+    var displayVoice = voice || 'server default';
-      return;
+    var text = 'Hello, this is a preview of the ' + displayVoice + ' voice. This is how your read-aloud feature will sound.';
-    }
-
-    // Temporarily save preference, generate preview, then restore
-    var text = 'Hello, this is a preview of the ' + voice + ' voice. This is how your read-aloud feature will sound.';
 
     var btnPreview = document.getElementById('btn-preview-voice');
     if (btnPreview) {
@@ -173,11 +169,11 @@
       btnPreview.innerHTML = '<i class="fas fa-spinner fa-spin"></i> Loading...';
     }
 
-    // Save current preference temporarily
+    // Save current preference temporarily (or clear it if "server default" selected)
     fetch('/api/user/preferences', {
       method: 'POST',
       headers: getAuthHeaders(),
-      body: JSON.stringify({ tts_voice: voice })
+      body: JSON.stringify({ tts_voice: voice || null })
     })
     .then(function() {
       // Generate audio with new voice