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/Icon.qml

@@ -0,0 +1,14 @@
+import QtQuick
+
+// Material icon rendered once per (name, color, size) by the icon provider.
+Image {
+    property string name: "music"
+    property color color: Theme.textHi
+    property int size: 20
+    width: size
+    height: size
+    sourceSize: Qt.size(size * Screen.devicePixelRatio, size * Screen.devicePixelRatio)
+    source: Theme.icon(name, color)
+    smooth: true
+    asynchronous: false
+}