webapp: device manager: optimize cards for tab nav

the top border of the card was breaking the design of the tabs, where
the active tab would be "visually connected" to the content. also, the
rounded border at the top did not blend in with the navbar's bottom
border.

webapp/src/scss/styles.scss

@@ -57,3 +57,10 @@ div.card.card-table table .value {
     text-align: right;
     padding-right: 0;
 }
+
+// blend cards into tabbed navigation (device admin view)
+div.card-tabbed {
+    border-top: 0;
+    border-top-left-radius: 0;
+    border-top-right-radius: 0;
+}

webapp/src/views/DeviceAdminView.vue

@@ -51,7 +51,7 @@
                     aria-labelledby="nav-pin-tab"
                     tabindex="0"
                 >
-                    <div class="card">
+                    <div class="card card-tabbed">
                         <div class="card-body">
                             <div class="row mb-3">
                                 <label for="inputPinProfile" class="col-sm-2 col-form-label">{{
@@ -115,7 +115,7 @@
                     aria-labelledby="nav-display-tab"
                     tabindex="0"
                 >
-                    <div class="card">
+                    <div class="card card-tabbed">
                         <div class="card-body">
                             <InputElement
                                 :label="$t('deviceadmin.PowerSafe')"
@@ -216,7 +216,7 @@
                     aria-labelledby="nav-leds-tab"
                     tabindex="0"
                 >
-                    <div class="card">
+                    <div class="card card-tabbed">
                         <div class="card-body">
                             <InputElement
                                 :label="$t('deviceadmin.EqualBrightness')"