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/chain_studio_flow_editor.dart

@@ -40,6 +40,8 @@ export 'src/run_driver.dart'
     show
         FlowRunDriver,
         FlowRunEvent,
+        PickedFileData,
+        PickFileCallback,
         StepStarted,
         StepCompleted,
         StepFailed,