Add localStorage helpers for battery display settings

frontend/src/utils/batteryDisplay.ts

@@ -0,0 +1,42 @@
+const PERCENT_KEY = 'remoteterm-show-battery-percent';
+const VOLTAGE_KEY = 'remoteterm-show-battery-voltage';
+
+export function getShowBatteryPercent(): boolean {
+  try {
+    return localStorage.getItem(PERCENT_KEY) === 'true';
+  } catch {
+    return false;
+  }
+}
+
+export function setShowBatteryPercent(enabled: boolean): void {
+  try {
+    if (enabled) {
+      localStorage.setItem(PERCENT_KEY, 'true');
+    } else {
+      localStorage.removeItem(PERCENT_KEY);
+    }
+  } catch {
+    // localStorage may be unavailable
+  }
+}
+
+export function getShowBatteryVoltage(): boolean {
+  try {
+    return localStorage.getItem(VOLTAGE_KEY) === 'true';
+  } catch {
+    return false;
+  }
+}
+
+export function setShowBatteryVoltage(enabled: boolean): void {
+  try {
+    if (enabled) {
+      localStorage.setItem(VOLTAGE_KEY, 'true');
+    } else {
+      localStorage.removeItem(VOLTAGE_KEY);
+    }
+  } catch {
+    // localStorage may be unavailable
+  }
+}