fix: sync refactor — fix 6 bugs + cache refresh na sync

- setDeletedAt update nu ook updatedAt (bug 2)
- getAllMeasurements filtert deletedAt IS NULL, nieuwe getAllMeasurementsForSync voor sync
- mergeMeasurements detecteert server deleted:true → deletedAt (bug 1)
- syncAll: pull-push split met remoteUpdatedMap, geen double-push (bug 3)
- hard delete alleen na API-succes (bug 4)
- DashboardScreen: refreshCache() na syncAll zodat UI niet op Laden blijft hangen
- deleteMeasurement() dead code verwijderd (bug 5)
- tests: 3 nieuwe testcases voor deleted → deletedAt mapping, 15/15 pass

src/utils/toast.ts

@@ -0,0 +1,26 @@
+import { Platform, ToastAndroid } from 'react-native'
+
+let _toastQueue: Array<{message: string; duration: number}> = []
+let _showFn: ((message: string, duration: number) => void) | null = null
+
+export function setToastShowFn(fn: (message: string, duration: number) => void) {
+  _showFn = fn
+}
+
+export function showToast(message: string, duration = 3000) {
+  if (_showFn) {
+    _showFn(message, duration)
+    return
+  }
+  if (Platform.OS === 'android') {
+    ToastAndroid.show(message, duration > 2000 ? ToastAndroid.LONG : ToastAndroid.SHORT)
+  }
+}
+
+export function showSuccess(message: string) {
+  showToast(message, 2500)
+}
+
+export function showError(message: string) {
+  showToast(message, 4000)
+}