[auth] Fix missing token (#6971)

## Description
When using Auth without backup, it was giving a error `Offline key is
missing`

**Reason**: During the `init` of `BaseConfiguration` if the `tokenKey`
is not set, we clear all the keys in the secure storage, and in this
process the `offlineAuthSecretKey` was also getting cleared

**Fix** Fixed by skipping the deletion of `offlineAuthSecretKey`  

## Tests
[Test Video](https://wormhole.app/qz3mol#Dlhr0NRpVQVQsrid2X-quA)

mobile/packages/configuration/lib/base_configuration.dart

@@ -35,6 +35,7 @@ class BaseConfiguration {
   static const userIDKey = "user_id";
   static const endPointKey = "endpoint";
   static const lastTempFolderClearTimeKey = "last_temp_folder_clear_time";
+  static const offlineAuthSecretKey = "offline_auth_secret_key";
 
   final kTempFolderDeletionTimeBuffer = const Duration(days: 1).inMicroseconds;
 
@@ -52,6 +53,11 @@ class BaseConfiguration {
 
   String? _volatilePassword;
 
+  // Keys that should not be deleted during logout
+  // These keys are necessary for functionality that needs to work even when users
+  // aren't signed in, such as using Auth without backup
+  List<String> preservedKeys = [offlineAuthSecretKey];
+
   Future<void> init(List<EnteBaseDatabase> dbs) async {
     _databases = dbs;
     _documentsDirectory = (await getApplicationDocumentsDirectory()).path;
@@ -62,13 +68,13 @@ class BaseConfiguration {
         accessibility: KeychainAccessibility.first_unlock_this_device,
       ),
     );
-    _setupKeys();
-    _setupFolders();
+    await _setupKeys();
+    await _setupFolders();
   }
 
   Future<void> logout({bool autoLogout = false}) async {
     await _preferences.clear();
-    _secureStorage.deleteAll();
+    await resetSecureStorage();
     for (final db in _databases) {
       await db.clearTable();
     }
@@ -78,6 +84,16 @@ class BaseConfiguration {
     Bus.instance.fire(SignedOutEvent());
   }
 
+  Future<void> resetSecureStorage() async {
+    // Delete all keys except preserved ones
+    final allKeys = await _secureStorage.readAll();
+    for (final key in allKeys.keys) {
+      if (!preservedKeys.contains(key)) {
+        await _secureStorage.delete(key: key);
+      }
+    }
+  }
+
   Future<KeyGenResult> generateKey(String password) async {
     // Create a master key
     final masterKey = CryptoUtil.generateKey();
@@ -217,7 +233,7 @@ class BaseConfiguration {
       if (split.length != mnemonicKeyWordCount) {
         String wordThatIsFollowedByEmptySpaceInSplit = '';
         for (int i = 0; i < split.length; i++) {
-          String word = split[i];
+          final String word = split[i];
           if (word.isEmpty) {
             wordThatIsFollowedByEmptySpaceInSplit =
                 '\n\nExtra space after word at position $i';
@@ -382,7 +398,7 @@ class BaseConfiguration {
   Future<void> _setupKeys() async {
     try {
       if (!_preferences.containsKey(tokenKey)) {
-        await _secureStorage.deleteAll();
+        await resetSecureStorage();
         return;
       }
       _key = await _secureStorage.read(key: keyKey);