fix(app): saved files are the own user's alone, and iOS's copy goes

On the desktop a saved file (a recovery phrase, recovery codes, an
archive with progress) is made the own user's alone (0600, chmod(2)
through the C library already in the process) on the emptied file before
anything is written, so no other user of the machine reads it; Windows'
files keep their folder's rights. On iOS, file_picker (darwin v2.1.2)
writes a copy into the app's temporary folder for the system's picker and
never removes it, saved or cancelled: it goes once the picker is done.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Signed-off-by: Tizian 「ティツィアーン」 <raisondetredev@pm.me>

packages/halkyon_app/test/files/file_saver_test.dart

@@ -0,0 +1,46 @@
+import 'dart:io';
+import 'dart:typed_data';
+
+import 'package:flutter_test/flutter_test.dart';
+import 'package:halkyon_app/halkyon_app.dart';
+import 'package:path/path.dart' as p;
+
+void main() {
+  late Directory folder;
+
+  setUp(() async {
+    folder = await Directory.systemTemp.createTemp('file_saver_test');
+  });
+  tearDown(() => folder.delete(recursive: true));
+
+  test("a saved file is the own user's alone where the platform has modes, "
+      'set before anything is written', () async {
+    final path = p.join(folder.path, 'halkyon-recovery-codes.txt');
+    // One there before, readable by everyone.
+    await File(path).writeAsString('old');
+    if (!Platform.isWindows) {
+      await Process.run('chmod', ['644', path]);
+    }
+
+    await writePrivately(path, Uint8List.fromList([1, 2, 3]));
+
+    expect(await File(path).readAsBytes(), [1, 2, 3]);
+    if (Platform.isLinux || Platform.isMacOS) {
+      expect(File(path).statSync().mode & 0x1ff, 0x180, reason: '0600');
+    }
+  }, skip: Platform.isWindows ? 'no modes' : false);
+
+  test("the system picker's copy in the temporary folder is removed, "
+      'others left', () async {
+    final copy = File(p.join(folder.path, 'halkyon-recovery-phrase.txt'));
+    final other = File(p.join(folder.path, 'something-else.txt'));
+    await copy.writeAsString('the phrase');
+    await other.writeAsString('other');
+
+    await forgetPickerCopy('halkyon-recovery-phrase.txt', temporary: folder);
+    await forgetPickerCopy('not-there.txt', temporary: folder);
+
+    expect(copy.existsSync(), isFalse);
+    expect(other.existsSync(), isTrue);
+  });
+}