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/CaptionButton.qml

@@ -0,0 +1,31 @@
+import QtQuick
+import QtQuick.Controls.Basic
+
+// Win7-flavoured caption button; close turns glossy red on hover.
+AbstractButton {
+    id: b
+    property string iconName: "minimize"
+    property bool danger: false
+    implicitWidth: 46
+    implicitHeight: 30
+    hoverEnabled: true
+    focusPolicy: Qt.NoFocus
+    background: Rectangle {
+        radius: 6
+        gradient: Gradient {
+            GradientStop { position: 0; color: b.danger && b.hovered ? "#F0645A" : (b.hovered ? Qt.rgba(1, 1, 1, 0.14) : "transparent") }
+            GradientStop { position: 0.5; color: b.danger && b.hovered ? "#D8392B" : (b.hovered ? Qt.rgba(1, 1, 1, 0.07) : "transparent") }
+            GradientStop { position: 1; color: b.danger && b.hovered ? "#C42B1C" : (b.hovered ? Qt.rgba(1, 1, 1, 0.05) : "transparent") }
+        }
+        border.color: b.hovered ? (b.danger ? "#FF8A80" : Qt.rgba(1, 1, 1, 0.14)) : "transparent"
+        Rectangle {
+            anchors.fill: parent
+            radius: parent.radius
+            color: b.down ? Qt.rgba(0, 0, 0, 0.2) : "transparent"
+        }
+        Glow { radius: 6; sigma: 5; color: "#FF5C4C"; opacity: b.danger && b.hovered ? 0.5 : 0 }
+    }
+    contentItem: Item {
+        Icon { anchors.centerIn: parent; name: b.iconName; size: 16; color: b.hovered ? "#FFFFFF" : Theme.textHi }
+    }
+}