Move and adjust variable controlling size of audio RX waterfall.

Closes recently-opened gap between control UI and waterfall.

firmware/application/ui_receiver.hpp

@@ -363,6 +363,8 @@ public:
 	void focus() override;
 
 private:
+	static constexpr ui::Dim header_height = 2 * 16;
+
 	RSSI rssi {
 		{ 21 * 8, 0, 6 * 8, 4 },
 	};