app: default the admin validation_method to manual

manual (we re-ran the agent ourselves) is the common case for the
maintainer team, so preselect it on the promote radio.

app.py

@@ -558,6 +558,7 @@ to publish the resulting row on the public leaderboard.
         admin_selection_md = gr.Markdown("_No rows selected._")
         admin_method_radio = gr.Radio(
             choices=list(VALID_METHODS),
+            value="manual",
             label="validation_method (applied to all rows on promote)",
             interactive=False,
         )