refactor: Calendar header style

Author: Jasenkoo

src/Vue3DatePicker/components/Calendar.vue

@@ -27,6 +27,7 @@
                     <div class="dp__calendar_header_item" v-if="weekNumbers">{{ weekNumName }}</div>
                     <div class="dp__calendar_header_item" v-for="(dayVal, i) in weekDays" :key="i">{{ dayVal }}</div>
                 </div>
+                <div class="dp__calendar_header_separator"></div>
                 <div class="dp__calendar">
                     <div class="dp__calendar_row" v-for="(week, weekInd) in mappedDates" :key="weekInd">
                         <div v-if="weekNumbers" class="dp__calendar_item dp__week_num">

src/Vue3DatePicker/style/components/_Calendar.scss

@@ -13,7 +13,7 @@
     align-items: center;
     color: var(--dp-text-color);
     white-space: nowrap;
-    border-bottom: thin solid var(--dp-border-color);
+    font-weight: bold;
 }
 
 .dp__calendar_header_item {
@@ -154,3 +154,9 @@
     @extend %__dp_range_preview;
     border-right: 1px dashed var(--dp-primary-color);
 }
+
+.dp__calendar_header_separator {
+    width: 100%;
+    height: 1px;
+    background: var(--dp-border-color);
+}