Remove placeholder prop from Monthpicker component

The `placeholder` prop was removed from the Monthpicker component as it was redundant. The input field now uses a default placeholder of 'yyyy-mm' directly within the MonthPicker.Input. Additionally, a new function getEksisterendeVerdi was introduced to handle specific period logic for field names including 'navArbeidsforholdPeriode'.

apps/dolly-frontend/src/main/js/src/components/ui/form/inputs/monthpicker/Monthpicker.tsx

@@ -14,7 +14,6 @@ interface MonthpickerProps {
 	onChange?: (date: Date) => void
 	minDate?: Date
 	maxDate?: Date
-	placeholder?: string
 }
 
 export const Monthpicker = ({
@@ -25,10 +24,18 @@ export const Monthpicker = ({
 	onChange,
 	minDate = null,
 	maxDate = null,
-	placeholder = null,
 }: MonthpickerProps) => {
 	const formMethods = useFormContext()
-	const eksisterendeVerdi = formMethods.watch(name)
+	const val = formMethods.watch(name)
+
+	function getEksisterendeVerdi() {
+		if (name.includes('navArbeidsforholdPeriode')) {
+			return val?.year ? new Date(val?.year, val?.monthValue) : null
+		}
+		return val
+	}
+
+	const eksisterendeVerdi = getEksisterendeVerdi()
 
 	const formattedDate =
 		eksisterendeVerdi instanceof Date
@@ -61,12 +68,7 @@ export const Monthpicker = ({
 		<InputWrapper size={'small'}>
 			<Label name={name} label={label}>
 				<MonthPicker {...monthpickerProps} dropdownCaption={true} selected={formattedDate}>
-					<MonthPicker.Input
-						label={null}
-						size={'small'}
-						placeholder={placeholder || 'yyyy-mm'}
-						{...inputProps}
-					/>
+					<MonthPicker.Input label={null} size={'small'} placeholder={'yyyy-mm'} {...inputProps} />
 				</MonthPicker>
 			</Label>
 		</InputWrapper>