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/CaptionButton.qml

@@ -0,0 +1,18 @@
+import QtQuick
+
+Item {
+    id: c
+    property string icon
+    property bool danger: false
+    property string tip: ""
+    signal clicked()
+    width: icon === "pane" ? 40 : 46
+    height: parent.height
+    Rectangle {
+        anchors.fill: parent
+        color: cm.pressed ? (c.danger ? "#F1707A" : X.greenDark) : cm.containsMouse ? (c.danger ? "#E81123" : X.greenHover) : "transparent"
+    }
+    Ico { anchors.centerIn: parent; name: c.icon; ink: "#FFFFFF"; size: c.icon === "pane" ? 16 : 24 }
+    MouseArea { id: cm; anchors.fill: parent; hoverEnabled: true; onClicked: c.clicked() }
+    Tip { target: cm; text: c.tip }
+}