fix: re-encode audio to AAC instead of copy for mp4 compatibility

-c copy may produce mp4 files with AC3/DTS/FLAC audio that macOS
AVPlayer cannot decode, resulting in silent playback.
Change to -c:v copy -c:a aac -b:a 192k to preserve video quality
while ensuring universally playable audio.

Sources/StreamRecorderKit/FFmpegRecorder.swift

@@ -56,7 +56,9 @@ public final class FFmpegRecorder: StreamRecorderEngine {
             args += ["-t", String(duration)]
         }
         args += [
-            "-c", "copy",
+            "-c:v", "copy",
+            "-c:a", "aac",
+            "-b:a", "192k",
             "-movflags", "+faststart",
             outputPath
         ]