feat(#38): Add periodic local storage backup for application form

legalconsenthub/app/composables/useLocalStorageBackup.ts

@@ -0,0 +1,97 @@
+import type { ApplicationFormDto } from '~~/.api-client'
+import { useLogger } from '~/composables/useLogger'
+
+interface LocalStorageBackup {
+  formId: string
+  sectionIndex: number
+  timestamp: number
+  formData: ApplicationFormDto
+}
+
+const STORAGE_KEY_PREFIX = 'lch-form-backup-'
+
+export function useLocalStorageBackup(formId: string) {
+  const storageKey = `${STORAGE_KEY_PREFIX}${formId}`
+  const logger = useLogger().withTag('localStorageBackup')
+
+  const hasBackup = ref(false)
+  const backupTimestamp = ref<Date | null>(null)
+  const backupSectionIndex = ref<number | null>(null)
+
+  function checkForBackup(): boolean {
+    if (import.meta.server) return false
+
+    try {
+      const storedApplicationForm = localStorage.getItem(storageKey)
+      if (storedApplicationForm) {
+        const backup = JSON.parse(storedApplicationForm) as LocalStorageBackup
+        hasBackup.value = true
+        backupTimestamp.value = new Date(backup.timestamp)
+        backupSectionIndex.value = backup.sectionIndex
+        return true
+      }
+    } catch {
+      clearBackup()
+    }
+    return false
+  }
+
+  function saveBackup(formData: ApplicationFormDto, currentSectionIndex: number): void {
+    if (import.meta.server) return
+
+    try {
+      const backup: LocalStorageBackup = {
+        formId,
+        sectionIndex: currentSectionIndex,
+        timestamp: Date.now(),
+        formData
+      }
+      localStorage.setItem(storageKey, JSON.stringify(backup))
+      hasBackup.value = true
+      backupTimestamp.value = new Date(backup.timestamp)
+      backupSectionIndex.value = currentSectionIndex
+    } catch (e) {
+      // localStorage might be full or unavailable
+      logger.error('Error saving backup', e)
+    }
+  }
+
+  function loadBackup(): ApplicationFormDto | null {
+    if (import.meta.server) return null
+
+    try {
+      const stored = localStorage.getItem(storageKey)
+      if (stored) {
+        const backup = JSON.parse(stored) as LocalStorageBackup
+        return backup.formData
+      }
+    } catch {
+      clearBackup()
+    }
+    return null
+  }
+
+  function clearBackup(): void {
+    if (import.meta.server) return
+
+    localStorage.removeItem(storageKey)
+    hasBackup.value = false
+    backupTimestamp.value = null
+    backupSectionIndex.value = null
+  }
+
+  // Call synchronously - import.meta.server guard handles SSR safety.
+  // Do NOT use onMounted here: this composable is called after `await` in async setup(),
+  // which means Vue's active component instance is gone and lifecycle hooks cannot be registered.
+  checkForBackup()
+
+  return {
+    hasBackup: readonly(hasBackup),
+    backupTimestamp: readonly(backupTimestamp),
+    backupSectionIndex: readonly(backupSectionIndex),
+    saveBackup,
+    loadBackup,
+    clearBackup,
+    checkForBackup
+  }
+}