give the selection cards their own parts instead of label blobs

src/ui/selection/EmptyNote.h

@@ -0,0 +1,15 @@
+#pragma once
+
+#include <QString>
+#include <QLabel>
+
+// A dimmed aside explaining why a part of the card is not there yet -- a construction
+// site's "no buffers until built" (REQ-UI-SELECTION-CARD). Styled apart from the card's
+// values so it reads as an explanation rather than as data.
+class EmptyNote : public QLabel
+{
+    Q_OBJECT
+
+public:
+    explicit EmptyNote(const QString& text, QWidget* parent = nullptr);
+};