feat(site-builder): Element/Site tabs in the right panel

Refs #36

craft/src/styles/editor.css

@@ -2184,3 +2184,21 @@ body {
     bottom: calc(var(--mobile-tabbar-height) + env(safe-area-inset-bottom, 0px) + 12px);
   }
 }
+
+/* #36 right panel tabs */
+.right-panel-placeholder {
+  padding: 24px 12px;
+  text-align: center;
+  color: var(--color-text-muted);
+  font-size: 13px;
+  line-height: 1.5;
+}
+
+.right-panel-placeholder p {
+  margin: 0 0 8px;
+}
+
+.right-panel-placeholder-hint {
+  font-size: 12px;
+  opacity: 0.8;
+}