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/IconButton.qml

@@ -0,0 +1,47 @@
+import QtQuick
+import QtQuick.Controls.Basic
+
+// Round icon button; checkable variant used for solo / mute.
+AbstractButton {
+    id: b
+    property string iconName: "music"
+    property color iconColor: Theme.textLo
+    property color activeColor: Theme.primary
+    property color hoverColor: Theme.alpha(Theme.textHi, 0.08)
+    property int size: 32
+    property int iconSize: 18
+    property string tip: ""
+    property bool glow: true
+    implicitWidth: size
+    implicitHeight: size
+    hoverEnabled: true
+    focusPolicy: Qt.TabFocus
+    opacity: enabled ? 1 : 0.35
+
+    background: Item {
+        Glow {
+            radius: Math.round(b.size / 2)
+            sigma: 4
+            color: b.checked ? b.activeColor : Theme.accentGlow
+            opacity: b.glow && (b.hovered || b.visualFocus) && b.enabled ? 0.35 : 0
+        }
+        Rectangle {
+            anchors.fill: parent
+            radius: width / 2
+            color: b.checked ? Theme.alpha(b.activeColor, b.down ? 0.34 : 0.22)
+                             : (b.down ? Theme.alpha(Theme.textHi, 0.14) : (b.hovered ? b.hoverColor : "transparent"))
+            border.width: b.checked ? 1 : 0
+            border.color: Theme.alpha(b.activeColor, 0.6)
+            Behavior on color { ColorAnimation { duration: Theme.fast } }
+        }
+    }
+    contentItem: Item {
+        Icon {
+            anchors.centerIn: parent
+            name: b.iconName
+            size: b.iconSize
+            color: b.checked ? b.activeColor : (b.hovered ? Theme.textHi : b.iconColor)
+        }
+    }
+    Tip { visible: b.hovered && b.tip.length > 0; text: b.tip }
+}