StemRoller: desktop stem separator, Android stems player, and server daemons

- stemroller/: PySide6/QML desktop app and player for separating music into stems
- android/: Plex music player that plays per-stem mixes, with Android Auto support,
  a graphic + parametric (Equalizer APO-style) EQ, and offline downloads
- server/: stemserve (serves stems to the app) and stemd (separates the library on
  the server's GPU, with "Stem Now" priority requests)

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

stemroller/player/qml/StemPlayer/XCheck.qml

@@ -0,0 +1,24 @@
+import QtQuick
+
+// Office checkbox: square box, green tick.
+Item {
+    id: c
+    property bool checked: false
+    property string text: ""
+    signal toggled()
+    implicitWidth: 18 + lbl.implicitWidth + 4
+    implicitHeight: 20
+    opacity: enabled ? 1 : 0.4
+    Rectangle {
+        id: box
+        width: 13
+        height: 13
+        y: (parent.height - height) / 2
+        x: 2
+        color: "#FFFFFF"
+        border.color: ma.containsMouse ? X.green : "#7A7A7A"
+        Ico { visible: c.checked; anchors.centerIn: parent; name: "check"; size: 12 }
+    }
+    T { id: lbl; x: 20; height: parent.height; text: c.text }
+    MouseArea { id: ma; anchors.fill: parent; hoverEnabled: true; onClicked: c.toggled() }
+}