iOS: degrade gracefully where the Android-only native services are missing

Share/pick/sync calls and streams no longer throw MissingPluginException at
startup; upload/download/sync-now show 'X isn't available on this platform
yet'. Offline/sync mirror dir no longer uses the Android-only external dir.
Info.plist: Face ID and photo library usage strings.

Co-Authored-By: Claude Sonnet 5.5 <noreply@anthropic.com>

lib/services/download_service.dart

@@ -38,6 +38,9 @@ class DownloadService {
           .toList(),
     );
 
-    await _channel.invokeMethod('startDownload', {...args, 'files': filesJson});
+    await invokeOrExplain(_channel, 'startDownload', 'Downloading', {
+      ...args,
+      'files': filesJson,
+    });
   }
 }