Store JWTs in secure storage instead of a plaintext Hive box

**Problem**

`Hive.openBox` is called with no `encryptionCipher`, so the access and refresh tokens sit in plaintext in `placementAuthBox.hive` under the app's Documents directory, which iOS includes in iCloud and iTunes backups by default.

The naming actively misleads: the opener is `_openEncryptedBox()` and the writers are `_encrypt()` and `_encryptToken()`, none of which encrypt anything.

**Where**

- `lib/services/auth/auth_service.dart:91-97` (`_openEncryptedBox`, plain `Hive.openBox`)
- `lib/services/auth/auth_service.dart:77-80` (`_encrypt`)
- `lib/services/auth/auth_service.dart:87-89` (`_encryptToken`)

**Expected**

Tokens at rest in the Keychain on iOS and EncryptedSharedPreferences on Android.

**Fix sketch**

- Replace the Hive box with `flutter_secure_storage` for the token pair.
- Rename the three methods to describe what they actually do.
- Drop `hive` and `hive_flutter` from `pubspec.yaml` once nothing else uses them; this box is currently their only use.

Start with lib/services/auth/auth_service.dart, especially _openEncryptedBox(), _encrypt(), and _encryptToken(), then inspect pubspec.yaml to confirm Hive is only used here. Verify the token pair is stored using the expected iOS and Android secure-storage backends, the methods have accurate names, and unused Hive dependencies are removed.

Written by the indexing model from the issue text.