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/XScrollBar.qml

@@ -0,0 +1,18 @@
+import QtQuick
+import QtQuick.Controls.Basic
+
+// Office scrollbar: flat gray thumb on a light track.
+ScrollBar {
+    id: sb
+    policy: ScrollBar.AlwaysOn
+    minimumSize: 0.04
+    padding: 2
+    implicitWidth: 15
+    implicitHeight: 15
+    contentItem: Rectangle {
+        implicitWidth: 9
+        implicitHeight: 9
+        color: sb.pressed ? "#7A7A7A" : sb.hovered ? "#A6A6A6" : "#C2C2C2"
+    }
+    background: Rectangle { color: "#F0F0F0"; border.color: "#E0E0E0" }
+}