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

frontend/src/test/batteryDisplay.test.ts

@@ -0,0 +1,44 @@
+import { describe, expect, it } from 'vitest';
+import { mvToPercent, formatBatteryLabel } from '../utils/batteryDisplay';
+
+describe('mvToPercent', () => {
+  it('clamps to 100 above table ceiling', () => {
+    expect(mvToPercent(4500)).toBe(100);
+    expect(mvToPercent(4190)).toBe(100);
+  });
+
+  it('clamps to 0 below table floor', () => {
+    expect(mvToPercent(3100)).toBe(0);
+    expect(mvToPercent(2800)).toBe(0);
+  });
+
+  it('returns exact table values at boundaries', () => {
+    expect(mvToPercent(4050)).toBe(90);
+    expect(mvToPercent(3630)).toBe(40);
+  });
+
+  it('interpolates between table entries', () => {
+    // Midpoint between 3630 (40%) and 3720 (50%) = 3675 → ~45%
+    const mid = mvToPercent(3675);
+    expect(mid).toBeGreaterThan(40);
+    expect(mid).toBeLessThan(50);
+  });
+});
+
+describe('formatBatteryLabel', () => {
+  it('returns null when both toggles are off', () => {
+    expect(formatBatteryLabel(4050, false, false)).toBeNull();
+  });
+
+  it('returns percentage only', () => {
+    expect(formatBatteryLabel(4050, true, false)).toBe('90%');
+  });
+
+  it('returns voltage only', () => {
+    expect(formatBatteryLabel(4050, false, true)).toBe('4050mV');
+  });
+
+  it('returns combined when both enabled', () => {
+    expect(formatBatteryLabel(4050, true, true)).toBe('90% (4050mV)');
+  });
+});