fix: close Raw Email dialog automatically after download (#114)

After a successful download, Navigator.pop is called so the dialog
dismisses without requiring a manual close. Adds a widget test that
verifies this using a fake PathProviderPlatform and IOOverrides so the
entire async chain runs as pure microtasks inside the Flutter test zone.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

pubspec.yaml

@@ -72,6 +72,7 @@ dev_dependencies:
   test: ^1.25.0
   mockito: ^5.4.4
   fake_async: ^1.3.1
+  path_provider_platform_interface: ^2.1.2
   sqlite3: any  # used directly in test/unit/db_test_helper.dart
   url_launcher_platform_interface: ^2.3.2
   plugin_platform_interface: ^2.1.8