feat: implement Sleepy sleep diary

lib/sleep.test.ts

@@ -0,0 +1,18 @@
+import { describe, expect, it } from 'vitest';
+import { calculateMetrics, formatDuration } from './sleep';
+
+describe('sleep calculations', () => {
+  it('calculates overnight time in bed, total sleep, and efficiency', () => {
+    expect(calculateMetrics({
+      bedtime: '23:40',
+      riseTime: '07:10',
+      sleepLatencyMinutes: 30,
+      wasoMinutes: 40,
+    })).toEqual({ timeInBedMinutes: 450, totalSleepMinutes: 380, efficiency: 84 });
+  });
+
+  it('formats a duration for the summary', () => {
+    expect(formatDuration(380)).toBe('6 ч 20 мин');
+    expect(formatDuration(5)).toBe('5 мин');
+  });
+});