Secure Local Data Storage

Background: Native [iOS / Android] app, [SwiftUI / Compose]. I store this
sensitive data on-device: [tokens / credentials / personal data / cached
user content]. Currently stored in: [Keychain/Keystore / UserDefaults/
SharedPrefs / database / files].

Use case: Store each piece of data in the right place with the right
protection—and stop storing sensitive data in the wrong place.

Implementation details:
- iOS: Keychain for secrets/tokens (with the appropriate accessibility
  attribute, e.g., afterFirstUnlock), Data Protection classes for files,
  and what must NOT go in UserDefaults. Android: Keystore-backed keys,
  EncryptedSharedPreferences / encrypted DataStore, and what must NOT go
  in plain SharedPreferences.
- Encryption at rest for sensitive databases/files
- Classifying each data item by sensitivity and mapping to a store
- Clearing sensitive data on logout and handling backup exclusion (so
  secrets don't leak via device backups)

Deliverable: A data-sensitivity classification mapped to the correct
storage mechanism per platform, the secure-storage implementation for the
most sensitive item, the logout-clear behavior, and the backup-exclusion
settings.