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/qml/StemRoller/Toast.qml

@@ -0,0 +1,65 @@
+import QtQuick
+
+// Material snackbar on surface-2.
+Item {
+    id: t
+    property string message: ""
+    property string kind: "info"
+    property string folder: ""
+    property real maxWidth: 800
+    width: Math.min(maxWidth, row.implicitWidth + 32)
+    height: 48
+    opacity: 0
+    visible: opacity > 0
+    y: shown ? 0 : 12
+    property bool shown: false
+
+    function show(m, k, f) {
+        message = m; kind = k; folder = f
+        shown = true
+        hideTimer.restart()
+    }
+    Timer { id: hideTimer; interval: 5000; onTriggered: t.shown = false }
+    states: State { when: t.shown; PropertyChanges { t.opacity: 1 } }
+    Behavior on opacity { NumberAnimation { duration: Theme.med; easing.type: Easing.BezierSpline; easing.bezierCurve: Theme.ease } }
+    Behavior on y { NumberAnimation { duration: Theme.slow; easing.type: Easing.BezierSpline; easing.bezierCurve: Theme.ease } }
+
+    Rectangle {
+        anchors.fill: parent
+        radius: 8
+        color: Theme.surface2
+        border.color: t.kind === "error" ? Theme.alpha(Theme.error, 0.5) : t.kind === "success" ? Theme.alpha(Theme.success, 0.45) : Theme.outline
+        Elevation { level: 3; radius: 8 }
+    }
+    Row {
+        id: row
+        anchors { left: parent.left; leftMargin: 16; verticalCenter: parent.verticalCenter }
+        spacing: 12
+        Icon {
+            anchors.verticalCenter: parent.verticalCenter
+            name: t.kind === "error" ? "error" : t.kind === "success" ? "check" : "music"
+            size: 18
+            color: t.kind === "error" ? Theme.error : t.kind === "success" ? Theme.success : Theme.accentGlow
+        }
+        Txt {
+            anchors.verticalCenter: parent.verticalCenter
+            text: t.message
+            width: Math.min(implicitWidth, 640)
+        }
+        AeroButton {
+            visible: t.folder.length > 0
+            anchors.verticalCenter: parent.verticalCenter
+            variant: "text"
+            pillHeight: 32
+            text: "Open folder"
+            onClicked: { app.openFolder(t.folder); t.shown = false }
+        }
+        IconButton {
+            anchors.verticalCenter: parent.verticalCenter
+            size: 28; iconSize: 14
+            iconName: "close"
+            glow: false
+            onClicked: t.shown = false
+        }
+    }
+}