Track server clock offset for usage data

frontend/src/features/usage/composables/__tests__/useServerClock.spec.ts

@@ -0,0 +1,42 @@
+import { describe, expect, it } from 'vitest'
+import { calculateServerClockOffsetMs, useServerClock } from '../useServerClock'
+
+describe('useServerClock', () => {
+  it('calculates offset from the request midpoint', () => {
+    const offset = calculateServerClockOffsetMs({
+      server_now_unix_ms: 10_500,
+      client_send_unix_ms: 20_000,
+      client_receive_unix_ms: 20_200,
+    })
+
+    expect(offset).toBe(-9_600)
+  })
+
+  it('ignores missing or invalid timing samples', () => {
+    expect(calculateServerClockOffsetMs(undefined)).toBeNull()
+    expect(calculateServerClockOffsetMs({
+      server_now_unix_ms: Number.NaN,
+      client_send_unix_ms: 20_000,
+      client_receive_unix_ms: 20_200,
+    })).toBeNull()
+    expect(calculateServerClockOffsetMs({
+      server_now_unix_ms: 10_500,
+      client_send_unix_ms: 20_200,
+      client_receive_unix_ms: 20_000,
+    })).toBeNull()
+  })
+
+  it('keeps the previous offset when a response has no server timing', () => {
+    const clock = useServerClock()
+
+    clock.updateServerClockOffset({
+      server_now_unix_ms: 10_500,
+      client_send_unix_ms: 20_000,
+      client_receive_unix_ms: 20_200,
+    })
+    clock.updateServerClockOffset(undefined)
+
+    expect(clock.hasServerClockOffset.value).toBe(true)
+    expect(clock.serverClockOffsetMs.value).toBe(-9_600)
+  })
+})