feat: add demo mode for testing without BLE device

Add "Demo Mode" button in device scan screen that bypasses BLE
connection and calibration. NIRS monitoring generates simulated
random data at 4Hz. Both monitoring views show DEMO badge in top bar.

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

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

@@ -22,6 +22,7 @@ class AppState(private val context: Context) {
     var sensorMode by mutableStateOf(SensorMode.PIEZO)
     var bladderLevel by mutableStateOf(BladderLevel())
     var isDeviceConnected by mutableStateOf(false)
+    var isDemoMode by mutableStateOf(false)
     var piezoSettings by mutableStateOf(PiezoSettings())
     var catheterCount = mutableIntStateOf(0)
 
@@ -66,11 +67,22 @@ class AppState(private val context: Context) {
     fun goHome() {
         NirsManager.instance.stopStreaming()
         NirsManager.instance.reset()
-        bleManager.disconnect()
+        if (!isDemoMode) bleManager.disconnect()
         isDeviceConnected = false
+        isDemoMode = false
         currentScreen = AppScreen.HOME
     }
 
+    fun enterDemoMode() {
+        isDemoMode = true
+        isDeviceConnected = true
+        currentScreen = if (sensorMode == SensorMode.NIRS) {
+            AppScreen.VIVAMYO_MONITORING
+        } else {
+            AppScreen.PIEZO_MONITORING
+        }
+    }
+
     fun startFromHome() {
         currentScreen = AppScreen.SENSOR_SELECT
     }