feat: debug recording off by default, toggle in settings

- AudioPipeline.debugRecording defaults to false (was true)
- SettingsRepository: persist debug_recording preference
- CallViewModel: debugRecording StateFlow + setter, wired to AudioPipeline
- Only records PCM + RMS when explicitly enabled in settings

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

android/app/src/main/java/com/wzp/audio/AudioPipeline.kt

@@ -57,7 +57,7 @@ class AudioPipeline(private val context: Context) {
     /** Whether to attach hardware AEC. Must be set before start(). */
     var aecEnabled: Boolean = true
     /** Enable debug recording of PCM + RMS histogram to cache dir. */
-    var debugRecording: Boolean = true
+    var debugRecording: Boolean = false
     private var captureThread: Thread? = null
     private var playoutThread: Thread? = null