Replace the GnuCash import checkbox column with native multi-select

Selecting bookings to import now uses the Treeview's own multi-selection
(click, Ctrl+click, Shift+click for ranges) instead of a dedicated
checkbox column that had to be clicked precisely -- more standard and
much faster for marking many rows at once.

Bookings matching an existing payment's date+amount are no longer
blocked from selection; they're still flagged (red row, "Bereits
vorhanden"). If any selected booking is such a duplicate, importing now
asks whether to skip those or instead adopt the booking's description
onto the already-recorded payment. That relabeling is handled by a new
repository.update_payment_reference, which only touches the reference
and gnucash_transaction_id fields, leaving date/amount/allocations
untouched.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

tests/test_contributions.py

@@ -251,6 +251,71 @@ def test_payment_can_be_deleted_with_its_allocations(tmp_path) -> None:
     assert repository.get_events(member.member_id)[-1].event_type == "payment_deleted"
 
 
+def test_payment_reference_can_be_replaced_without_touching_amount_or_allocations(tmp_path) -> None:
+    repository, member = _repository_with_claim(tmp_path)
+    payment = repository.record_payment(
+        member.member_id,
+        "claim-1",
+        payment_date="2026-06-21",
+        amount="10.00",
+        allocation_amount="10.00",
+        reference="Alte Referenz",
+    )
+
+    updated = repository.update_payment_reference(
+        member.member_id,
+        payment["payment_id"],
+        reference="Neue Referenz aus GnuCash",
+        gnucash_transaction_id="TX-99",
+    )
+
+    data = repository.get_contributions(member.member_id)
+    assert updated["reference"] == "Neue Referenz aus GnuCash"
+    assert updated["gnucash_transaction_id"] == "TX-99"
+    assert data.payments[0]["amount"] == "10.00"
+    assert payment_allocated_total(data, payment["payment_id"]) == Decimal("10.00")
+    assert repository.get_events(member.member_id)[-1].event_type == "payment_changed"
+
+
+def test_payment_reference_update_rejects_gnucash_id_already_used_elsewhere(tmp_path) -> None:
+    repository, member = _repository_with_claim(tmp_path)
+    repository.record_payment(
+        member.member_id,
+        "claim-1",
+        payment_date="2026-06-21",
+        amount="10.00",
+        allocation_amount="10.00",
+        gnucash_transaction_id="TX-1",
+    )
+    data = repository.get_contributions(member.member_id)
+    data.claims.append(
+        {
+            "claim_id": "claim-2",
+            "claim_key": "second-claim",
+            "title": "Zweite Forderung",
+            "amount": "20.00",
+            "due_date": "2026-12-31",
+            "status": "open",
+        }
+    )
+    repository.save_contributions(member.member_id, data)
+    second_payment = repository.record_payment(
+        member.member_id,
+        "claim-2",
+        payment_date="2026-06-22",
+        amount="20.00",
+        allocation_amount="20.00",
+    )
+
+    with pytest.raises(RepositoryError, match="GnuCash-ID bereits verwendet"):
+        repository.update_payment_reference(
+            member.member_id,
+            second_payment["payment_id"],
+            reference="Duplikatversuch",
+            gnucash_transaction_id="TX-1",
+        )
+
+
 def test_credit_claim_settlement_is_displayed_as_positive_amount() -> None:
     claim = {"claim_id": "claim-1", "title": "Kautionsrückzahlung", "amount": "-25.00"}
     data = ContributionData(