Add some tests and move the helpers into their own TS file

frontend/src/utils/batteryDisplay.ts

@@ -1,5 +1,44 @@
 export const BATTERY_DISPLAY_CHANGE_EVENT = 'remoteterm-battery-display-change';
 
+// Meshtastic default OCV table (meshtastic/firmware src/power.h)
+const OCV_TABLE: [number, number][] = [
+  [4190, 100],
+  [4050, 90],
+  [3990, 80],
+  [3890, 70],
+  [3800, 60],
+  [3720, 50],
+  [3630, 40],
+  [3530, 30],
+  [3420, 20],
+  [3300, 10],
+  [3100, 0],
+];
+
+export function mvToPercent(mv: number): number {
+  if (mv >= OCV_TABLE[0][0]) return 100;
+  if (mv <= OCV_TABLE[OCV_TABLE.length - 1][0]) return 0;
+  for (let i = 0; i < OCV_TABLE.length - 1; i++) {
+    const [highMv, highPct] = OCV_TABLE[i];
+    const [lowMv, lowPct] = OCV_TABLE[i + 1];
+    if (mv >= lowMv)
+      return Math.round(lowPct + ((mv - lowMv) / (highMv - lowMv)) * (highPct - lowPct));
+  }
+  return 0;
+}
+
+export function formatBatteryLabel(
+  mv: number,
+  showPercent: boolean,
+  showVoltage: boolean
+): string | null {
+  if (!showPercent && !showVoltage) return null;
+  const pct = mvToPercent(mv);
+  if (showPercent && showVoltage) return `${pct}% (${mv}mV)`;
+  if (showPercent) return `${pct}%`;
+  return `${mv}mV`;
+}
+
 const PERCENT_KEY = 'remoteterm-show-battery-percent';
 const VOLTAGE_KEY = 'remoteterm-show-battery-voltage';