feat: MVP-5 Sprint 1 - Backup/Restore system

- Add backup types and RecallBackup format
- Create backup snapshot engine (createBackupSnapshot)
- Add IndexedDB storage for local backups
- Implement retention policy (max 10, 30-day cleanup)
- Add backup validation and restore logic (merge/replace modes)
- Add backup restore UI dialog with preview and confirmation
- Add unsaved changes guard hook
- Integrate backups section in Settings
- Add backup endpoint to export-import API

src/hooks/use-unsaved-changes.ts

@@ -0,0 +1,19 @@
+import { useEffect } from 'react'
+
+export function useUnsavedChanges(
+  isDirty: boolean,
+  message = '¿Salir sin guardar cambios?'
+) {
+  useEffect(() => {
+    if (!isDirty) return
+
+    const handleBeforeUnload = (e: BeforeUnloadEvent) => {
+      e.preventDefault()
+      e.returnValue = message
+      return message
+    }
+
+    window.addEventListener('beforeunload', handleBeforeUnload)
+    return () => window.removeEventListener('beforeunload', handleBeforeUnload)
+  }, [isDirty, message])
+}