fix: Seahorse does not properly work on many Linux devices

- stop use of `flutter_secure_storage` for Linux in favor of unencrypted
  storage

Unlike many other platforms, many Linux distributions have built-in
support for FDE or home directory encryption. As long as
`flutter_secure_storage` makes FluffyChat useles on many Linuxes, this
seems to be the only solution until they stop using Seahorse.

Signed-off-by: TheOneWithTheBraid <the-one@with-the-braid.cf>

lib/utils/matrix_sdk_extensions.dart/fluffybox_database.dart

@@ -40,6 +40,8 @@ class FlutterFluffyBoxDatabase extends FluffyBoxDatabase {
       final containsEncryptionKey =
           await secureStorage.containsKey(key: _cipherStorageKey);
       if (!containsEncryptionKey) {
+        // do not try to create a buggy secure storage for new Linux users
+        if (Platform.isLinux) throw MissingPluginException();
         final key = Hive.generateSecureKey();
         await secureStorage.write(
           key: _cipherStorageKey,

lib/utils/matrix_sdk_extensions.dart/flutter_matrix_hive_database.dart

@@ -31,7 +31,7 @@ class FlutterMatrixHiveStore extends FamedlySdkHiveDatabase {
     HiveCipher hiverCipher;
     try {
       // Workaround for secure storage is calling Platform.operatingSystem on web
-      if (kIsWeb) throw MissingPluginException();
+      if (kIsWeb || Platform.isLinux) throw MissingPluginException();
 
       const secureStorage = FlutterSecureStorage();
       final containsEncryptionKey =