Integrate secure storage service

finlog_app/fluttery/lib/secure_storage.dart

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+import 'package:fluttery/fluttery.dart';
+
+/// Interface for secure storage operations.
+///
+/// Provides methods for securely storing and retrieving sensitive data
+/// like passwords, tokens, API keys, etc. Data stored through this interface
+/// is encrypted and stored in the device's secure storage (Keychain on iOS,
+/// Keystore on Android).
+abstract class SecureStorage implements Service {
+  /// Stores a string value securely with the given [key].
+  ///
+  /// Returns a Future that completes when the value is successfully stored.
+  /// Throws an exception if the storage operation fails.
+  Future<void> write(String key, String value);
+
+  /// Retrieves the securely stored string value for the given [key].
+  ///
+  /// Returns the stored value if found, null otherwise.
+  /// Throws an exception if the retrieval operation fails.
+  Future<String?> read(String key);
+
+  /// Stores an integer value securely with the given [key].
+  ///
+  /// The integer is converted to a string for storage.
+  Future<void> writeInt(String key, int value);
+
+  /// Retrieves the securely stored integer value for the given [key].
+  ///
+  /// Returns the stored integer if found and valid, null otherwise.
+  Future<int?> readInt(String key);
+
+  /// Stores a boolean value securely with the given [key].
+  ///
+  /// The boolean is converted to a string for storage.
+  Future<void> writeBool(String key, bool value);
+
+  /// Retrieves the securely stored boolean value for the given [key].
+  ///
+  /// Returns the stored boolean if found and valid, null otherwise.
+  Future<bool?> readBool(String key);
+
+  /// Stores a double value securely with the given [key].
+  ///
+  /// The double is converted to a string for storage.
+  Future<void> writeDouble(String key, double value);
+
+  /// Retrieves the securely stored double value for the given [key].
+  ///
+  /// Returns the stored double if found and valid, null otherwise.
+  Future<double?> readDouble(String key);
+
+  /// Removes the securely stored value for the given [key].
+  ///
+  /// Returns a Future that completes when the value is successfully removed.
+  Future<void> delete(String key);
+
+  /// Removes all securely stored values.
+  ///
+  /// Returns a Future that completes when all values are successfully removed.
+  /// Use with caution as this operation cannot be undone.
+  Future<void> deleteAll();
+
+  /// Returns all keys currently stored in secure storage.
+  ///
+  /// Useful for debugging or migration purposes.
+  Future<Set<String>> readAllKeys();
+
+  /// Checks if a value exists for the given [key].
+  ///
+  /// Returns true if a value exists, false otherwise.
+  Future<bool> containsKey(String key);
+}