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

test/utils/battery_utils_test.dart

@@ -0,0 +1,23 @@
+import 'package:flutter_test/flutter_test.dart';
+import 'package:meshcore_open/utils/battery_utils.dart';
+
+void main() {
+  group('battery utils', () {
+    test('nmc range maps 3.0V to 0% and 4.2V to 100%', () {
+      expect(estimateBatteryPercentFromVolts(3.0, 'nmc'), 0);
+      expect(estimateBatteryPercentFromVolts(4.2, 'nmc'), 100);
+    });
+
+    test('lifepo4 range maps 2.6V to 0% and 3.65V to 100%', () {
+      expect(estimateBatteryPercentFromVolts(2.6, 'lifepo4'), 0);
+      expect(estimateBatteryPercentFromVolts(3.65, 'lifepo4'), 100);
+    });
+
+    test('unknown chemistry falls back to nmc mapping', () {
+      expect(
+        estimateBatteryPercentFromMillivolts(3600, 'unknown'),
+        estimateBatteryPercentFromMillivolts(3600, 'nmc'),
+      );
+    });
+  });
+}