ESP32 link indicators, trip export with V2X + RSSI, trip/CSV cleanup

UI:
- Top bar shows a USB glyph reflecting the serial link on the ESP32-C5 path,
  instead of a Wi-Fi glyph driven by an MQTT state that is permanently
  disconnected there.
- Recording screen's OBU stream row follows whichever transport the selected
  hardware uses. It previously read as offline throughout a recording that was
  actively beaconing CAMs.
- Live map uses distinct markers: a centred dot for own position (a fact about
  the viewer, not a tracked object) and a teardrop pin for remote stations,
  tinted by alert severity. Both were osmdroid's identical default pin before,
  and severity required tapping a marker to read its label.
- Sensor Monitor moves out of the bottom nav to Settings > Developer. A live
  phone-sensor feed is a bench tool; the Dashboard already reports GNSS/IMU
  health. Screen and route are unchanged, just not in the rider's way.

Detection engine on the ESP32 path:
- Seed our own StationID from the persisted value CamTransmitLoop transmits.
  It was null here (the CiT One learns it from v2x/rx/obu_gnss, which doesn't
  exist on this path), so the self-heard-TX filter never fired: the ESP32 runs
  promiscuous for raw TX to work at all, hears our own CAMs back off the air,
  and they were tracked as a remote station - a ghost vehicle on top of the ego
  position, fed to the engine as a collision partner for itself.

RSSI:
- The firmware has always sent per-frame RSSI in byte 0 of every CAM_RX frame;
  the app discarded it. Now carried on Cam, persisted per V2X message, shown per
  station in the received-CAM list, and exported. Null for own CAMs and the
  whole CiT One path, neither of which has a measurement.

Trips, CSV and export (schema v3 -> v4):
- trips.sessionId links a trip to the CSV session recorded alongside it. The two
  are written by independent subsystems that the Recording button happens to
  start together; without the link, deleting a trip orphaned its CSV forever.
  Timestamp matching was rejected - close recordings would delete the wrong file.
- Deleting a trip now removes the CSV file AND its sessions row, so it stops
  appearing in the Session Log pointing at nothing.
- Per-trip combined CSV export: GPS track, detected events, V2X messages (with
  RSSI) and the raw sensor samples in one file, keyed by a leading type column.
  One file rather than a zip of tables because the point of the export is
  correlating those streams, and splitting them pushes the join downstream.
- Export takes the Activity context. Sharing from the ViewModel's Application
  context threw AndroidRuntimeException on startActivity - this crashed on the
  first tap of the share button.

Trips recorded before v4 have a null sessionId, so their exports omit raw sensor
rows and their CSVs still need clearing by hand once.

app/src/main/java/com/hawhamburg/micr0bu/MainActivity.kt

@@ -28,6 +28,9 @@ import androidx.navigation.compose.NavHost
 import androidx.navigation.compose.composable
 import androidx.navigation.compose.rememberNavController
 import androidx.navigation.navArgument
+import com.hawhamburg.micr0bu.data.mqtt.MqttConnectionState
+import com.hawhamburg.micr0bu.data.transport.UsbSerialState
+import com.hawhamburg.micr0bu.data.transport.ObuHardware
 import com.hawhamburg.micr0bu.ui.components.StatusTopBar
 import com.hawhamburg.micr0bu.ui.navigation.BottomNavBar
 import com.hawhamburg.micr0bu.ui.navigation.Screen
@@ -122,7 +125,14 @@ class MainActivity : AppCompatActivity() {
                 }
 
                 Scaffold(
-                    topBar = { StatusTopBar(state, mqttConnectionState) },
+                    topBar = {
+                        StatusTopBar(
+                            state = state,
+                            mqttConnectionState = mqttConnectionState,
+                            isEsp32 = obuHardware == ObuHardware.ESP32_C5,
+                            usbSerialState = usbSerialState,
+                        )
+                    },
                     bottomBar = { BottomNavBar(navController) },
                 ) { innerPadding ->
                     NavHost(
@@ -165,12 +175,19 @@ class MainActivity : AppCompatActivity() {
                             RecordingScreen(
                                 state = state,
                                 mqttConnectionState = mqttConnectionState,
+                                obuConnected = if (obuHardware == ObuHardware.ESP32_C5)
+                                    usbSerialState == UsbSerialState.CONNECTED
+                                else
+                                    mqttConnectionState == MqttConnectionState.CONNECTED,
                                 tripServiceState = tripServiceState,
                                 showBatteryOptPrompt = showBatteryOptPrompt,
                                 onToggleRecording = {
-                                    // Start / stop both CSV recording and the event detection service
+                                    // Start / stop both CSV recording and the event detection
+                                    // service. Order matters on start: the CSV session must exist
+                                    // before the trip row is written, so the trip can store its
+                                    // id and later delete the CSV along with itself.
                                     viewModel.toggleRecording()
-                                    tripViewModel.toggleRecording()
+                                    tripViewModel.toggleRecording(viewModel.activeSessionId)
                                 },
                                 onOpenSessionLog = {
                                     navController.navigate(Screen.Log.route)
@@ -294,6 +311,7 @@ class MainActivity : AppCompatActivity() {
                             DeveloperSettingsScreen(
                                 state           = state,
                                 onDeveloperMode = viewModel::setDeveloperMode,
+                                onOpenSensorMonitor = { navController.navigate(Screen.Sensors.route) },
                                 onBack          = { navController.popBackStack() },
                             )
                         }