fix(tts): filter null/undefined voices to handle Brave anti-fingerprinting

Brave browser's anti-fingerprinting user-script (makeFakeVoiceFromVoice)
intercepts the SpeechSynthesis voices array and crashes with
'undefined is not an object (evaluating Object.getPrototypeOf(voice))'
when iterating over null voice entries.

Defensive fix: filter null/undefined/no-lang entries from getVoices()
before processing, so Brave's proxy never receives invalid objects.

Fixes #58

assets/js/app.js

@@ -12371,7 +12371,7 @@ function _initBrowserTtsVoices(selectedVoice) {
     sel.innerHTML = '<option value="">— Caricamento voci… —</option>';
 
     const populate = () => {
-        const voices = window.speechSynthesis.getVoices();
+        const voices = (window.speechSynthesis.getVoices() || []).filter(v => v != null && v.lang);
         if (!voices.length) return false;
         // Italian voices first, then others
         const it = voices.filter(v => v.lang.startsWith('it'));