Fix crash from zero-length audio buffers in speech recognition

Guard against empty audio buffers before appending to speech
recognition request — AVAudioBuffer asserts non-zero data size.

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

Conjuga/Conjuga/Services/PronunciationService.swift

@@ -86,6 +86,7 @@ final class PronunciationService {
             }
 
             inputNode.installTap(onBus: 0, bufferSize: 1024, format: recordingFormat) { buffer, _ in
+                guard buffer.frameLength > 0 else { return }
                 request.append(buffer)
             }