feat(run): host-injected native file picker for flow file inputs

The Run tab asked the operator to type an absolute path into a text
field — with no file dialog anywhere. FlowEditorPage now accepts an
onPickFile callback (PickedFileData: name + bytes) that hosts wire
to their native picker; the path-entry dialog remains only as the
no-host fallback. 0.23.0.

Signed-off-by: flemming-it <sf@flemming.it>

lib/src/run_driver.dart

@@ -19,6 +19,19 @@
 
 import 'dart:typed_data';
 
+/// A file chosen by the host's native picker for a flow file input.
+class PickedFileData {
+  final String fileName;
+  final Uint8List bytes;
+  const PickedFileData({required this.fileName, required this.bytes});
+}
+
+/// Host-side native file picker. Returns null when the operator
+/// cancels. When a host does not supply one, the Run tab falls back
+/// to its manual path-entry dialog.
+typedef PickFileCallback = Future<PickedFileData?> Function();
+
+
 abstract class FlowRunDriver {
   /// Trigger a run of [flowName] with the supplied inputs.
   /// Text + file inputs come in separate maps so the host