feat(battery): warn about a very low transmitter battery in settings and on the home screen

G6SensorKitUI/G6CGMManager+UI.swift

@@ -117,6 +117,18 @@ extension G6CGMManager: CGMManagerUI {
             )
         }
 
+        // Ahead of calibration and signal loss: a dying battery is the likely
+        // cause of both, and replacing the transmitter is the actual fix. The
+        // highlight is the host's home-screen tag, so this is the one place
+        // the user sees it without opening settings.
+        if state.isBatteryVeryLow {
+            return G6StatusHighlight(
+                localizedMessage: LocalizedString("Transmitter Low", comment: "Status highlight when the transmitter battery is very low"),
+                imageName: "battery.0percent",
+                state: .warning
+            )
+        }
+
         if let reading = state.latestReading, reading.calibrationState.needsCalibration {
             return G6StatusHighlight(
                 localizedMessage: LocalizedString("Calibration Needed", comment: "Status highlight when a calibration is requested"),
@@ -162,7 +174,11 @@ extension G6CGMManager: CGMManagerUI {
         let progressState: DeviceLifecycleProgressState
         if remaining <= 0 {
             progressState = .critical
-        } else if remaining <= .hours(24) {
+        } else if remaining <= .hours(24) || state.isBatteryVeryLow {
+            // A dying transmitter battery counts too. Trio shows the status
+            // highlight ("Transmitter Low") on its home screen only while
+            // the progress is not normal, so without this it would stay
+            // hidden for most of a session.
             progressState = .warning
         } else {
             progressState = .normalCGM

G6SensorKitUI/Localizable.xcstrings

@@ -1,6 +1,10 @@
 {
   "sourceLanguage": "en",
   "strings": {
+    "Readings may be missing or arrive late, and the transmitter may respond slowly or not at all — including to stopping a session or calibrating. Replace the transmitter as soon as you can, and check your glucose with a fingerstick meter if readings stop.": {
+      "comment": "Settings banner body when the transmitter battery is very low",
+      "extractionState": "manual"
+    },
     "Sensor Failed": {
       "extractionState": "manual",
       "comment": "Status highlight for failed sensor"
@@ -49,6 +53,14 @@
       "extractionState": "manual",
       "comment": "Navigation title for pairing screen"
     },
+    "Transmitter Low": {
+      "comment": "Status highlight when the transmitter battery is very low",
+      "extractionState": "manual"
+    },
+    "Transmitter battery very low": {
+      "comment": "Settings banner title when the transmitter battery is very low",
+      "extractionState": "manual"
+    },
     "Warm-Up": {
       "extractionState": "manual",
       "comment": "Navigation title for warm-up screen"

G6SensorKitUI/Views/G6SettingsView.swift

@@ -292,11 +292,40 @@ struct G6SettingsView: View {
                         .foregroundStyle(.orange)
                 }
             }
+
+            // Last, so the session's own status reads first and the battery
+            // explains it underneath.
+            if viewModel.state.isBatteryVeryLow {
+                batteryVeryLowRow
+            }
         } header: {
             Text(LocalizedString("Sensor", comment: "Settings section: sensor"))
         }
     }
 
+    /// The Sensor section's last row. Stays up for as long as the battery
+    /// reads very low; there is nothing to dismiss, because the only fix is
+    /// a new transmitter. A tester ran on a near-dead battery for a day of
+    /// dropped connections and an End Sensor that never got through, with
+    /// nothing on this screen to explain why.
+    private var batteryVeryLowRow: some View {
+        HStack(alignment: .top, spacing: 12) {
+            Image(systemName: "battery.0percent")
+                .font(.title2)
+                .foregroundStyle(.red)
+            VStack(alignment: .leading, spacing: 4) {
+                Text(LocalizedString("Transmitter battery very low", comment: "Settings banner title when the transmitter battery is very low"))
+                    .font(.subheadline.weight(.semibold))
+                Text(LocalizedString("Readings may be missing or arrive late, and the transmitter may respond slowly or not at all — including to stopping a session or calibrating. Replace the transmitter as soon as you can, and check your glucose with a fingerstick meter if readings stop.", comment: "Settings banner body when the transmitter battery is very low"))
+                    .font(.footnote)
+                    .foregroundStyle(.secondary)
+                    .fixedSize(horizontal: false, vertical: true)
+            }
+        }
+        .padding(.vertical, 4)
+        .listRowBackground(Color.red.opacity(0.12))
+    }
+
     private var lastReadingSection: some View {
         Section(LocalizedString("Last Reading", comment: "Settings section: last reading")) {
             if let reading = viewModel.state.latestReading, !viewModel.state.isInWarmup {