refactor: Replace EncryptedSharedPreferences with DataStore + value encryption

[941kjw]

Summary

Replace EncryptedSharedPreferences with DataStore Preferences plus value-level encryption, aligning with current Android recommendations and improving reliability and async safety.

Current State

• Credentials, refresh interval, notifications, and reset times are stored via EncryptedSharedPreferences (androidx.security:security-crypto).
• All reads/writes are synchronous; main-thread use risks StrictMode and is discouraged.
• EncryptedSharedPreferences is deprecated as of security-crypto 1.1.0-alpha07; Google recommends DataStore + encryption for new code.

Proposed

Implement DataStore Preferences with in-app value encryption:

1. Storage

   • Use DataStore Preferences (androidx.datastore:datastore-preferences) for all secure preferences.
   • Retain value-level encryption for sensitive fields (tokens, OAuth client secret, etc.) using Android KeyStore (AES-256-GCM).

2. API shape

   • Suspend APIs: loadCredential, saveCredential, deleteCredential, deleteAllCredentials, setRefreshInterval, setNotificationsEnabled, saveResetTimes, loadResetTimes — expose as suspend and call from coroutines (ViewModel, Repository, Worker).
   • Sync-friendly APIs: hasCredential, getRefreshInterval, isNotificationsEnabled — back with an in-memory cache updated on first read and after writes so Tile/UI can call them without blocking.

3. Call sites

   • SettingsViewModel: Run load/save of credentials and settings inside viewModelScope.launch or suspend blocks.
   • QuotaRefreshWorker: Make checkForResets suspend and call loadResetTimes / saveResetTimes from doWork().
   • Repositories / DashboardViewModel / QuotaTileService: Repositories already use suspend; Tile can rely on cached hasCredential — no API change needed there.

4. Dependencies

   • Remove EncryptedSharedPreferences usage and androidx.security:security-crypto dependency.
   • Use existing datastore-preferences and standard KeyStore / Cipher for encryption.

References

• DataStore overview
• EncryptedSharedPreferences deprecation

Migration note

Existing data in the old EncryptedSharedPreferences file will not be auto-migrated (different format). After this change, users may need to re-enter credentials once in Settings.