feat(shell): active purchase downloads live in the sidebar, as a ring when compact

The Downloads section leaves the nav flyout. With the sidebar open, the
purchases downloading now sit under the Purchases entry, one row per
album with a thin progress bar, each reopening its detail. With the
sidebar collapsed, and in the compact header nav when it is closed, they
fold into one progress ring whose hover bubble lists every album with
its count and percentage. The ring is a Canvas that repaints only when a
publish moves the value — no timer, per the repaint-pulse rule. The
active set now carries done/total/percent per album.

cc

crates/qbz-qt/qml/shell/HeaderBar.qml

@@ -205,6 +205,27 @@ Rectangle {
     readonly property bool headerCompactOn: QbzShell.sidebarState === 2
         || (!QbzShell.navInSidebar && QbzShell.navHeaderCompact)
 
+    // Purchases downloading now (see Sidebar.qml's twin block).
+    readonly property var activeDownloads: {
+        try { return JSON.parse(QbzPurchases.activeDownloadsJson || "[]") } catch (e) { return [] }
+    }
+    readonly property real activeDownloadsOverall: {
+        var rows = root.activeDownloads
+        if (rows.length === 0) return 0
+        var sum = 0
+        for (var i = 0; i < rows.length; i++) sum += (rows[i].percent || 0)
+        return sum / rows.length / 100
+    }
+    readonly property string activeDownloadsSummary: {
+        var rows = root.activeDownloads
+        var lines = []
+        for (var i = 0; i < rows.length; i++)
+            lines.push((rows[i].title || QbzSession.tr("Purchases", QbzSession.trRev))
+                + " — " + (rows[i].done || 0) + "/" + (rows[i].total || 0)
+                + " · " + (rows[i].percent || 0) + "%")
+        return lines.join("\n")
+    }
+
     // Full text tab (HeaderBar.slint NavTab): 30px tall, radius sm, icon 14
     // (dropped under 1140px), label 11px — semibold + elevated fill when the
     // section is the current view.
@@ -464,6 +485,51 @@ Rectangle {
                 compact: true
                 anchors.verticalCenter: parent.verticalCenter
             }
+            // Purchases downloading now, as one ring (the sidebar is closed
+            // or the nav lives up here, so there is no row to list them in);
+            // the per-album detail is the hover bubble. Absent while idle.
+            Item {
+                visible: root.activeDownloads.length > 0
+                width: visible ? 30 : 0
+                height: 30
+                anchors.verticalCenter: parent.verticalCenter
+                QbzProgressRing {
+                    anchors.centerIn: parent
+                    size: 18
+                    value: root.activeDownloadsOverall
+                }
+                MouseArea {
+                    id: hdrRingArea
+                    anchors.fill: parent
+                    hoverEnabled: true
+                    cursorShape: Qt.PointingHandCursor
+                    onClicked: QbzShell.navigateTo("purchases")
+                }
+                ToolTip {
+                    visible: hdrRingArea.containsMouse
+                    delay: 0
+                    timeout: -1
+                    text: root.activeDownloadsSummary
+                    x: Math.round((parent.width - width) / 2)
+                    y: parent.height + 6
+                    contentItem: Text {
+                        text: root.activeDownloadsSummary
+                        color: theme.textPrimary
+                        font.pixelSize: 11
+                        font.weight: theme.weightMedium
+                        leftPadding: 9
+                        rightPadding: 9
+                        topPadding: 6
+                        bottomPadding: 6
+                    }
+                    background: Rectangle {
+                        color: theme.surfaceElevated
+                        radius: 4
+                        border.width: 1
+                        border.color: theme.borderMuted
+                    }
+                }
+            }
             // Thin separator + the playlists flyout button — ONLY while the
             // sidebar is really closed. In the opt-in always-compact mode the
             // sidebar list is still on screen, so the button stays away