qml: set BtcField.textAsSats initial value to match its text

textAsSats was being set to 0 initially, regardless of actual text, only healing after onTextChanged.

fixes https://github.com/spesmilo/electrum/issues/9974

electrum/gui/qml/components/controls/BtcField.qml

@@ -32,5 +32,5 @@ TextField {
         }
     }
 
-    Component.onCompleted: textAsSats = Config.unitsToSats('')
+    Component.onCompleted: amount.textChanged()
 }

electrum/gui/qml/qeinvoice.py

@@ -155,7 +155,7 @@ class QEInvoice(QObject, QtEventListener):
         return self._amountOverride
 
     @amountOverride.setter
-    def amountOverride(self, new_amount):
+    def amountOverride(self, new_amount: QEAmount):
         self._logger.debug(f'set new override amount {repr(new_amount)}')
         self._amountOverride.copyFrom(new_amount)
         self.amountOverrideChanged.emit()