Optimize voice note codec to 16 kHz / 20 kbps for smaller file sizes

- Reduce sample rate from 44.1 kHz to 16 kHz (telephony standard)
- Lower bitrate from 32 kbps to 20 kbps
- Results in ~37% file size reduction (~150 KB/min vs 240 KB/min)
- Increases max voice note length from 4.4 to 7 minutes over 1 MiB BLE limit
- Maintains excellent voice quality using native AAC-LC codec

bitchat/Features/voice/VoiceRecorder.swift

@@ -73,9 +73,9 @@ final class VoiceRecorder: NSObject, AVAudioRecorderDelegate {
             let outputURL = try makeOutputURL()
             let settings: [String: Any] = [
                 AVFormatIDKey: kAudioFormatMPEG4AAC,
-                AVSampleRateKey: 44_100,
+                AVSampleRateKey: 16_000,
                 AVNumberOfChannelsKey: 1,
-                AVEncoderBitRateKey: 32_000
+                AVEncoderBitRateKey: 20_000
             ]
 
             let audioRecorder = try AVAudioRecorder(url: outputURL, settings: settings)