feat: inline checkboxes and column dropdowns in match group editor

Replace separate checkbox row and "Customize columns" toggle with a
unified st.data_editor grid — Keep checkboxes at the start of each row,
differing columns render as inline selectbox dropdowns.

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

src/gui/app.py

@@ -215,6 +215,9 @@ if uploaded is not None:
 
                 def _clear_all():
                     st.session_state["review_decisions"] = {}
+                    for k in list(st.session_state):
+                        if k.startswith("editor_"):
+                            del st.session_state[k]
 
                 action_left, action_mid, action_right = st.columns(3)
                 with action_left: