feat(worker): server-authoritative Stopwatch screen with active-session recovery

apps/worker/src/lib/stopwatch.test.ts

@@ -0,0 +1,19 @@
+import { describe, expect, it } from 'vitest';
+import { elapsedSeconds, formatTime } from './stopwatch';
+
+describe('stopwatch logic', () => {
+  it('formats seconds as HH:MM:SS', () => {
+    expect(formatTime(0)).toBe('00:00:00');
+    expect(formatTime(65)).toBe('00:01:05');
+    expect(formatTime(3661)).toBe('01:01:01');
+  });
+
+  it('computes elapsed seconds since start excluding paused time', () => {
+    expect(elapsedSeconds(1000, 6000, 0)).toBe(5);
+    expect(elapsedSeconds(1000, 6000, 2000)).toBe(3);
+  });
+
+  it('never returns a negative elapsed value', () => {
+    expect(elapsedSeconds(5000, 1000, 0)).toBe(0);
+  });
+});