fix: sync deletion propagation, add base64 util, timeout, and merge improvements

Critical:
- Add deleted column: deleteMeasurement now soft-deletes, merge filters
  out deleted items based on updatedAt (was: deletions were resurrected)
- Replace btoa with UTF-8-safe encodeBase64 utility for Hermes compat

Important:
- Preserve createdAt when overwriting duplicate-date measurements
- Add 15s AbortController timeout to all WebDAV fetch calls
- Revert broken date prefilling regression (MeasurementForm)
- getAllMeasurements/latest/byDate now filter deleted records
- Replace Dimensions with useWindowDimensions for rotation awareness
- Replace silent catch with console.warn in HistoryScreen

Tests: 8 new tests for deletion merge cases and soft-delete queries

tests/syncService.test.ts

@@ -19,6 +19,7 @@ const base: Measurement = {
   createdAt: '2024-06-01T08:00:00.000Z',
   updatedAt: '2024-06-01T08:00:00.000Z',
   syncStatus: 'local',
+  deleted: 0,
 }
 
 describe('mergeMeasurements', () => {
@@ -54,4 +55,48 @@ describe('mergeMeasurements', () => {
     expect(result).toHaveLength(1)
     expect(result[0].weight).toBe(77)
   })
+
+  it('filters out locally deleted items even if present on remote', () => {
+    const local = { ...base, deleted: 1 as const, updatedAt: '2024-06-05T08:00:00.000Z' }
+    const remote = { ...base, deleted: 0 as const, updatedAt: '2024-06-03T08:00:00.000Z' }
+    const result = mergeMeasurements([local], [remote])
+    expect(result).toHaveLength(0)
+  })
+
+  it('filters out remotely deleted items when remote is newer', () => {
+    const local = { ...base, deleted: 0 as const, updatedAt: '2024-06-04T08:00:00.000Z' }
+    const remote = { ...base, deleted: 1 as const, updatedAt: '2024-06-05T08:00:00.000Z' }
+    const result = mergeMeasurements([local], [remote])
+    expect(result).toHaveLength(0)
+  })
+
+  it('keeps local active item when local is newer than remote deletion', () => {
+    const local = { ...base, deleted: 0 as const, weight: 78, updatedAt: '2024-06-06T08:00:00.000Z' }
+    const remote = { ...base, deleted: 1 as const, updatedAt: '2024-06-05T08:00:00.000Z' }
+    const result = mergeMeasurements([local], [remote])
+    expect(result).toHaveLength(1)
+    expect(result[0].weight).toBe(78)
+  })
+
+  it('keeps active item when local deleted is older than remote active', () => {
+    const local = { ...base, deleted: 1 as const, updatedAt: '2024-06-01T08:00:00.000Z' }
+    const remote = { ...base, deleted: 0 as const, updatedAt: '2024-06-05T08:00:00.000Z', syncStatus: 'synced' as const }
+    const result = mergeMeasurements([local], [remote])
+    expect(result).toHaveLength(1)
+    expect(result[0].weight).toBe(75)
+  })
+
+  it('merges multiple active and deleted items correctly', () => {
+    const localActive = { ...base, id: 'a', weight: 70, updatedAt: '2024-06-05T08:00:00.000Z' }
+    const localDeleted = { ...base, id: 'b', weight: 80, deleted: 1 as const, updatedAt: '2024-06-05T08:00:00.000Z' }
+    const remoteActive = { ...base, id: 'c', weight: 90, updatedAt: '2024-06-03T08:00:00.000Z', syncStatus: 'synced' as const }
+    const remoteDeleted = { ...base, id: 'd', weight: 100, deleted: 1 as const, updatedAt: '2024-06-05T08:00:00.000Z' }
+
+    const result = mergeMeasurements(
+      [localActive, localDeleted],
+      [remoteActive, remoteDeleted]
+    )
+    expect(result).toHaveLength(2)
+    expect(result.map(m => m.id).sort()).toEqual(['a', 'c'])
+  })
 })