qml: follow-up BtcField validator: take base unit fully into account

Not just for the fractional part, but also for the integer part.

follow-up 4df6052567

electrum/gui/qml/qeconfig.py

@@ -4,6 +4,7 @@ from typing import TYPE_CHECKING
 
 from PyQt5.QtCore import pyqtProperty, pyqtSignal, pyqtSlot, QObject, QRegularExpression
 
+from electrum.bitcoin import TOTAL_COIN_SUPPLY_LIMIT_IN_BTC
 from electrum.i18n import set_language, languages
 from electrum.logging import get_logger
 from electrum.util import DECIMAL_POINT_DEFAULT, base_unit_name_to_decimal_point
@@ -85,8 +86,11 @@ class QEConfig(AuthMixin, QObject):
     @pyqtProperty('QRegularExpression', notify=baseUnitChanged)
     def btcAmountRegex(self):
         decimal_point = base_unit_name_to_decimal_point(self.config.get_base_unit())
-        exp = '[0-9]{0,8}'
-        if decimal_point:
+        max_digits_before_dp = (
+            len(str(TOTAL_COIN_SUPPLY_LIMIT_IN_BTC))
+            + (base_unit_name_to_decimal_point("BTC") - decimal_point))
+        exp = '[0-9]{0,%d}' % max_digits_before_dp
+        if decimal_point > 0:
             exp += '\\.'
             exp += '[0-9]{0,%d}' % decimal_point
         return QRegularExpression(exp)

electrum/util.py

@@ -104,6 +104,7 @@ def decimal_point_to_base_unit_name(dp: int) -> str:
 
 
 def base_unit_name_to_decimal_point(unit_name: str) -> int:
+    """Returns the max number of digits allowed after the decimal point."""
     # e.g. "BTC" -> 8
     try:
         return base_units[unit_name]