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/RButton.qml

@@ -0,0 +1,59 @@
+import QtQuick
+
+// A ribbon command: large (icon over a label) or small (icon beside a label).
+Item {
+    id: b
+    property string icon: ""
+    property string text: ""
+    property bool big: false
+    property bool checkable: false
+    property bool checked: false
+    property bool dropdown: false
+    property string tip: ""
+    signal clicked()
+    implicitWidth: big ? Math.max(44, lbl.implicitWidth + 12) : (icon ? 22 : 6) + (text ? lbl.implicitWidth + 8 : 0) + (dropdown ? 12 : 0)
+    implicitHeight: big ? 66 : 22
+    opacity: enabled ? 1 : 0.4
+
+    Rectangle {
+        anchors.fill: parent
+        color: b.checked ? X.checked : ma.pressed ? X.pressed : ma.containsMouse ? X.hover : "transparent"
+        border.color: b.checked ? X.checkedBorder : "transparent"
+    }
+    Ico {
+        id: ic
+        visible: b.icon !== ""
+        name: b.icon
+        size: b.big ? 32 : 16
+        x: b.big ? (parent.width - width) / 2 : 3
+        y: b.big ? 4 : 3
+    }
+    T {
+        id: lbl
+        text: b.text
+        font.pixelSize: X.fsUi
+        x: b.big ? (parent.width - width) / 2 : (b.icon ? 23 : 4)
+        y: b.big ? 38 : 0
+        height: b.big ? implicitHeight : parent.height
+        width: b.big ? Math.min(implicitWidth, 90) : implicitWidth
+        horizontalAlignment: Text.AlignHCenter
+        wrapMode: b.big ? Text.WordWrap : Text.NoWrap
+        maximumLineCount: 2
+        elide: Text.ElideNone
+        lineHeight: 0.95
+    }
+    Ico {
+        visible: b.dropdown
+        name: "dropdown"
+        size: 10
+        x: b.big ? (parent.width - width) / 2 : parent.width - 12
+        y: b.big ? parent.height - 13 : 6
+    }
+    MouseArea {
+        id: ma
+        anchors.fill: parent
+        hoverEnabled: true
+        onClicked: b.clicked()
+    }
+    Tip { target: ma; text: b.tip || b.text }
+}