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>

src/ccma/storage/repository.py

@@ -1426,6 +1426,44 @@ class MemberRepository:
         )
         return payment
 
+    def update_payment_reference(
+        self,
+        member_id: str,
+        payment_id: str,
+        *,
+        reference: str,
+        gnucash_transaction_id: str = "",
+        actor_name: str = "Vorstand",
+    ) -> dict:
+        """Relabel an existing payment without touching its date, amount, or
+        allocations -- used e.g. when a GnuCash import recognizes a booking as
+        matching an already-recorded payment and the board wants to adopt the
+        (better) description from the statement instead of re-importing it."""
+        data = self.get_contributions(member_id)
+        payment = next(
+            (item for item in data.payments if str(item.get("payment_id", "")) == payment_id),
+            None,
+        )
+        if payment is None:
+            raise RepositoryError("Zahlung nicht gefunden.")
+        gnucash_id = gnucash_transaction_id.strip()
+        if gnucash_id and gnucash_id != str(payment.get("gnucash_transaction_id", "")):
+            self._assert_gnucash_id_available(gnucash_id, exclude_payment_id=payment_id)
+        payment["reference"] = reference.strip()
+        if gnucash_id:
+            payment["gnucash_transaction_id"] = gnucash_id
+        payment["updated_at"] = datetime.now().astimezone().isoformat(timespec="seconds")
+        self.save_contributions(member_id, data)
+        self.append_event(
+            member_id,
+            event_type="payment_changed",
+            summary=f"Zahlung geändert: Referenz aktualisiert ({payment['reference']})",
+            actor_type="user",
+            actor_name=actor_name,
+            references={"payment_id": payment_id},
+        )
+        return payment
+
     def delete_payment(self, member_id: str, payment_id: str) -> None:
         data = self.get_contributions(member_id)
         payment = next(