feat: add sync service with WebDAV merge logic

tests/syncService.test.ts

@@ -0,0 +1,57 @@
+import { mergeMeasurements } from '../src/sync/syncService'
+import { Measurement } from '../src/types'
+
+jest.mock('expo-sqlite', () => ({
+  openDatabaseAsync: jest.fn(),
+}))
+
+const base: Measurement = {
+  id: 'id-1',
+  weight: 75,
+  bmi: 22,
+  bodyFatPercentage: 15,
+  fatMass: 10,
+  musclePercentage: 40,
+  muscleMass: 30,
+  visceralFat: 8,
+  bmr: 1600,
+  measuredAt: '2024-06-01T08:00:00.000Z',
+  createdAt: '2024-06-01T08:00:00.000Z',
+  updatedAt: '2024-06-01T08:00:00.000Z',
+  syncStatus: 'local',
+}
+
+describe('mergeMeasurements', () => {
+  it('returns empty array when both are empty', () => {
+    expect(mergeMeasurements([], [])).toEqual([])
+  })
+
+  it('keeps local items not on remote', () => {
+    const result = mergeMeasurements([base], [])
+    expect(result).toHaveLength(1)
+    expect(result[0].id).toBe('id-1')
+  })
+
+  it('keeps remote items not on local', () => {
+    const remote = { ...base, syncStatus: 'synced' as const }
+    const result = mergeMeasurements([], [remote])
+    expect(result).toHaveLength(1)
+    expect(result[0].id).toBe('id-1')
+  })
+
+  it('picks the newer version when both have same id', () => {
+    const local = { ...base, weight: 75, updatedAt: '2024-06-02T08:00:00.000Z' }
+    const remote = { ...base, weight: 76, updatedAt: '2024-06-03T08:00:00.000Z' }
+    const result = mergeMeasurements([local], [remote])
+    expect(result).toHaveLength(1)
+    expect(result[0].weight).toBe(76)
+  })
+
+  it('picks local when local is newer', () => {
+    const local = { ...base, weight: 77, updatedAt: '2024-06-04T08:00:00.000Z' }
+    const remote = { ...base, weight: 76, updatedAt: '2024-06-03T08:00:00.000Z' }
+    const result = mergeMeasurements([local], [remote])
+    expect(result).toHaveLength(1)
+    expect(result[0].weight).toBe(77)
+  })
+})