Add OP_RETURN support for heirs

Allow heirs to produce an OP_RETURN output instead of a regular BTC
payment. An address prefixed with OP_RETURN: carries hex data (max 80
bytes); such heirs always have amount 0 and are excluded from amount
calculations. GUI dialog shows a message field with the decoded text,
the heir list displays the decoded message, and the OP_RETURN output
is emitted with zero value in inheritance transactions.

bal/gui/qt/lists.py

@@ -157,7 +157,17 @@ class HeirListWidget(MyTreeView, MessageBoxMixin):
             heir = self.bal_window.heirs[key]
             labels = [""] * len(self.Columns)
             labels[self.Columns.NAME] = key
-            labels[self.Columns.ADDRESS] = heir[0]
+            if is_op_return_address(heir[0]):
+                data_hex = heir[0][len(OP_RETURN_PREFIX):]
+                try:
+                    decoded = bytes.fromhex(data_hex).decode("utf-8", errors="replace")
+                    if len(decoded) > 40:
+                        decoded = decoded[:40] + "\u2026"
+                    labels[self.Columns.ADDRESS] = decoded
+                except Exception:
+                    labels[self.Columns.ADDRESS] = "OP_RETURN"
+            else:
+                labels[self.Columns.ADDRESS] = heir[0]
             labels[self.Columns.AMOUNT] = Util.decode_amount(
                 heir[1], self.decimal_point
             )