fix: prevent match group expanders from collapsing on button click

Replace st.rerun() with on_click callbacks so decisions write to
session state before the natural rerun. Decided groups auto-collapse
with status in the label; undecided groups stay expanded. Added undo
button on decided groups.

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

src/gui/app.py

@@ -213,11 +213,7 @@ if uploaded is not None:
                 # Individual group cards
                 decisions = st.session_state["review_decisions"]
                 for i, group in enumerate(result.match_groups):
-                    decision = match_group_card(group, df, group_num=i + 1)
-                    if decision is not None:
-                        decisions[group.group_id] = decision
-                        st.session_state["review_decisions"] = decisions
-                        st.rerun()
+                    match_group_card(group, df, group_num=i + 1)
 
                 # Show decision summary
                 if decisions: