fix: resolve UnknownBaseUnit crash in QML btcAmountRegex for non-BTC chains

Replace hardcoded \"BTC\" with get_base_units_list()[0] so the top-level
unit name is resolved dynamically from chain constants (e.g. \"BTCP\" for
BitcoinPurple), preventing the UnknownBaseUnit exception on receive screen.

electrum/gui/qml/qeconfig.py

@@ -7,7 +7,7 @@ from PyQt6.QtCore import pyqtProperty, pyqtSignal, pyqtSlot, QObject, QRegularEx
 from electrum.bitcoin import TOTAL_COIN_SUPPLY_LIMIT_IN_BTC
 from electrum.i18n import set_language, get_gui_lang_names
 from electrum.logging import get_logger
-from electrum.util import base_unit_name_to_decimal_point
+from electrum.util import base_unit_name_to_decimal_point, get_base_units_list
 from electrum.gui import messages
 
 from .qetypes import QEAmount
@@ -106,7 +106,7 @@ class QEConfig(AuthMixin, QObject):
         decimal_point = base_unit_name_to_decimal_point(self.config.get_base_unit())
         max_digits_before_dp = (
             len(str(TOTAL_COIN_SUPPLY_LIMIT_IN_BTC))
-            + (base_unit_name_to_decimal_point("BTC") - decimal_point))
+            + (base_unit_name_to_decimal_point(get_base_units_list()[0]) - decimal_point))
         exp = '^[0-9]{0,%d}' % max_digits_before_dp
         decimal_point += extra_precision
         if decimal_point > 0: