fix(status): align battery health algorithm with mole-mac

Mirror SystemMetricsCollector.batteryHealthPercent from mole-mac
b0f9e35b: prefer NominalChargeCapacity over AppleRawMaxCapacity,
compute the ratio against DesignCapacity with half-away-from-zero
rounding, clamp to [0, 100]. Reading the raw ioreg "MaxCapacity"
field was only normalized on Apple Silicon and undefined on Intel;
the new fields exist on all Macs and match the Mac app value, which
is what users compare against in System Settings.