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/lib/backup-validator.ts

@@ -0,0 +1,75 @@
+import { RecallBackup } from '@/types/backup'
+
+interface ValidationResult {
+  valid: boolean
+  errors: string[]
+  info?: {
+    noteCount: number
+    tagCount: number
+    createdAt: string
+    source: string
+  }
+}
+
+export function validateBackup(data: unknown): ValidationResult {
+  const errors: string[] = []
+
+  if (typeof data !== 'object' || data === null) {
+    return { valid: false, errors: ['Backup must be an object'] }
+  }
+
+  const backup = data as Record<string, unknown>
+
+  if (!validateSchemaVersion(backup as unknown as RecallBackup)) {
+    errors.push('Missing or invalid schemaVersion (expected "1.0")')
+  }
+
+  if (!backup.createdAt || typeof backup.createdAt !== 'string') {
+    errors.push('Missing or invalid createdAt field')
+  }
+
+  if (!backup.source || typeof backup.source !== 'string') {
+    errors.push('Missing or invalid source field')
+  }
+
+  if (!backup.metadata || typeof backup.metadata !== 'object') {
+    errors.push('Missing or invalid metadata field')
+  } else {
+    const metadata = backup.metadata as Record<string, unknown>
+    if (typeof metadata.noteCount !== 'number') {
+      errors.push('Missing or invalid metadata.noteCount')
+    }
+    if (typeof metadata.tagCount !== 'number') {
+      errors.push('Missing or invalid metadata.tagCount')
+    }
+  }
+
+  if (!backup.data || typeof backup.data !== 'object') {
+    errors.push('Missing or invalid data field')
+  } else {
+    const data = backup.data as Record<string, unknown>
+    if (!Array.isArray(data.notes)) {
+      errors.push('Missing or invalid data.notes (expected array)')
+    }
+  }
+
+  if (errors.length > 0) {
+    return { valid: false, errors }
+  }
+
+  const metadata = backup.metadata as { noteCount: number; tagCount: number }
+  return {
+    valid: true,
+    errors: [],
+    info: {
+      noteCount: metadata.noteCount,
+      tagCount: metadata.tagCount,
+      createdAt: backup.createdAt as string,
+      source: backup.source as string,
+    },
+  }
+}
+
+export function validateSchemaVersion(backup: RecallBackup): boolean {
+  return backup.schemaVersion === '1.0'
+}