feat: PlacementGuideView + MeasurementLogService + navigation flow

PlacementGuideView.kt:
- 6-sensor Canvas layout (CH0-CH3 vertical + CH4/CH5 lateral wings)
- Per-channel UrinAI detection (green/red color coding)
- Direction guidance (up/down/left/right)
- Check Position → maa → detect → display
- Start Measurement when 4+ channels detected

MeasurementLogService.kt:
- JSON Lines format, daily rotation
- Events: measurement, placement, ble_connected/disconnected, error
- 90-day auto-purge
- App internal storage: vesiscan_logs/YYYY-MM-DD.jsonl

AppState.kt + MainActivity.kt:
- PLACEMENT_GUIDE screen added to navigation
- Flow: personalization → placementGuide → piezoMonitoring

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

app/src/main/java/com/example/medilightv2android/MainActivity.kt

@@ -21,6 +21,7 @@ import com.example.medilightv2android.ui.views.calibration.CalibrationView
 import com.example.medilightv2android.ui.views.connection.DeviceScanView
 import com.example.medilightv2android.ui.views.history.MeasurementHistoryView
 import com.example.medilightv2android.ui.views.home.HomeView
+import com.example.medilightv2android.ui.views.monitoring.PlacementGuideView
 import com.example.medilightv2android.ui.views.monitoring.PiezoMonitoringView
 import com.example.medilightv2android.ui.views.monitoring.VivamyoMonitoringView
 import com.example.medilightv2android.ui.views.monitoring.UrineCameraScreen
@@ -74,6 +75,7 @@ fun MedilightApp(modifier: Modifier = Modifier) {
             AppScreen.DEVICE_SCAN -> DeviceScanView(appState)
             AppScreen.NIRS_CALIBRATION -> CalibrationView(appState)
             AppScreen.PIEZO_PERSONALIZATION -> PiezoPersonalizationView(appState)
+            AppScreen.PLACEMENT_GUIDE -> PlacementGuideView(appState)
             AppScreen.PIN_SETUP -> PinView(appState, PinMode.SETUP, context)
             AppScreen.PIN_ENTRY -> PinView(appState, PinMode.ENTER, context)
             AppScreen.VIVAMYO_MONITORING -> VivamyoMonitoringView(appState)