Fix repeater battery % inconsistency and add configurable repeater battery chemistry (#199)

* fix(repeater): unify battery percentage math and add repeater chemistry setting

- Add shared battery percent utility used by connector, repeater status, and telemetry

- Add repeater-specific battery chemistry persistence and service accessors

- Add repeater chemistry selector in Repeater Hub

- Ensure telemetry and status compute percentages consistently from same chemistry

- Add focused battery utility tests

Refs #116

Refs #174

* fix: Flutter Analyzer Errors fixed Recent Merge Compatible

* Unify repeater battery source across status and telemetry

lib/utils/battery_utils.dart

@@ -0,0 +1,26 @@
+typedef BatteryVoltageRange = ({int minMv, int maxMv});
+
+BatteryVoltageRange batteryVoltageRange(String chemistry) {
+  switch (chemistry) {
+    case 'lifepo4':
+      return (minMv: 2600, maxMv: 3650);
+    case 'lipo':
+      return (minMv: 3000, maxMv: 4200);
+    case 'nmc':
+    default:
+      return (minMv: 3000, maxMv: 4200);
+  }
+}
+
+int estimateBatteryPercentFromMillivolts(int millivolts, String chemistry) {
+  final range = batteryVoltageRange(chemistry);
+  if (millivolts <= range.minMv) return 0;
+  if (millivolts >= range.maxMv) return 100;
+  return (((millivolts - range.minMv) * 100) / (range.maxMv - range.minMv))
+      .round();
+}
+
+int estimateBatteryPercentFromVolts(double volts, String chemistry) {
+  final millivolts = (volts * 1000).round();
+  return estimateBatteryPercentFromMillivolts(millivolts, chemistry);
+}