refactor(ui): One QML file per screen; the HDMI boost's setup screens and backend commands removed

ui/qml/Btn.qml

@@ -0,0 +1,20 @@
+// A button with the input's button name in front.
+import QtQuick
+
+Rectangle {
+    id: btn
+    property string k
+    property string text
+    property bool primary: false
+    property bool focusRing: false
+    signal clicked
+    height: 48; radius: 12; width: row.implicitWidth + 36
+    color: primary ? Theme.accent : Theme.button
+    border.width: focusRing ? 2 : 0; border.color: Theme.textHi
+    Row { id: row; anchors.centerIn: parent; spacing: 10
+        Glyph { k: btn.k; visible: btn.k !== ""; dark: btn.primary; anchors.verticalCenter: parent.verticalCenter }
+        Text { text: btn.text; color: btn.primary ? Theme.ink : Theme.text; font.family: Theme.body; font.pixelSize: 15; font.weight: Font.DemiBold
+               anchors.verticalCenter: parent.verticalCenter }
+    }
+    MouseArea { anchors.fill: parent; cursorShape: Qt.PointingHandCursor; onClicked: btn.clicked() }
+}