Add battery chemistry and power state monitoring for repeaters

[rsp2k]

Summary

• Add per-repeater battery chemistry setting for accurate percentage estimates
• Support firmware-reported chemistry detection (companion: byte 11, repeater: byte 60)
• Add "no battery" option for wall-powered devices
• Add power state monitoring with USB/Solar/Charging detection and input voltage display
• Protocol bytes: 60=chemistry, 61=power_state flags, 62-63=input_mv

Power State Display

• USB icon (blue) when USB connected
• Solar icon (orange) when solar connected
• Charging icon (green) when charging
• Input voltage shown when external power detected

Test plan

• Verify firmware chemistry detection with lock icon
• Verify user-configurable dropdown for devices without firmware support
• Verify battery percentage recalculates on chemistry change
• Verify Power Source row displays icons correctly (tested on simulated firmware)
• Verify backward compatibility (row hidden on older firmware)

Addresses #116

[446564]

I love the idea but this recreates some existing functionality that should be pulled out into a helper and reused to remain consistent.

meshcore-open/lib/connector/meshcore_connector.dart

Lines 220 to 247 in 72f0aa7

	String _batteryChemistryForDevice() {
	final deviceId = _device?.remoteId.toString();
	if (deviceId == null || _appSettingsService == null) return 'nmc';
	return _appSettingsService!.batteryChemistryForDevice(deviceId);
	}
	int _estimateBatteryPercent(int millivolts, String chemistry) {
	final range = _batteryVoltageRange(chemistry);
	final minMv = range.$1;
	final maxMv = range.$2;
	if (millivolts <= minMv) return 0;
	if (millivolts >= maxMv) return 100;
	return (((millivolts - minMv) * 100) / (maxMv - minMv)).round();
	}
	(int, int) _batteryVoltageRange(String chemistry) {
	switch (chemistry) {
	case 'lifepo4':
	return (2600, 3650);
	case 'lipo':
	return (3000, 4200);
	case 'nmc':
	default:
	return (3000, 4200);
	}
	}
	List<Message> getMessages(Contact contact) {