refactor(qbz-qt): phase 23 part 1 — QML module layout + first five domain bridges

Two fronts of the modularization, both gated green before this commit.

QML leaves the flat qml/ directory for seven modules: cards, controls,
rows, settings, shell, theme, views. Only Main.qml and LoginScreen.qml
stay at the root. build.rs matches the tree exactly (55 files), relative
imports and asset paths were re-anchored for the new depth.

The QbzBridge God-object starts breaking up: QbzSession, QbzShell,
QbzPlayer, QbzQueue and QbzHome are now their own #[cxx_qt::bridge]
singletons, each with its own OnceLock<CxxQtThread> and ui() hop, booted
in order from Main.qml. Bridge files stay directly under src/ because
cxx-qt-build only accepts rust_files from one directory (QTBUG-93443).

bridge.rs keeps 5.5 domains (library, album/artist, settings,
integrations, search, playlist) and dies once they are split out.

cc

crates/qbz-qt/qml/controls/CardMenu.qml

@@ -0,0 +1,49 @@
+// CardMenu — the shared ⋯ menu surface for card overlays (promoted from
+// LibraryView in phase 21): 33px rows, icon 15 + label 13, driven by an
+// `entries` model of {label, icon, action}; emits picked(action).
+
+import QtQuick
+import com.blitzfc.qbz
+import "../theme"
+
+QbzContextMenu {
+    id: cmRoot
+
+    property var entries: []
+    signal picked(string action)
+
+    QbzTheme { id: theme }
+
+    Repeater {
+        model: cmRoot.entries
+        delegate: Rectangle {
+            required property var modelData
+            width: parent ? parent.width : 0
+            height: 33
+            radius: 5
+            color: cmiArea.containsMouse ? theme.surfaceHover : "transparent"
+            Row {
+                anchors.fill: parent
+                anchors.leftMargin: 8
+                spacing: 8
+                QbzIcon { name: modelData.icon; width: 15; height: 15; anchors.verticalCenter: parent.verticalCenter; tintName: "secondary" }
+                Text {
+                    height: parent.height
+                    width: parent.width - 23
+                    text: modelData.label
+                    color: theme.textSecondary
+                    font.pixelSize: 13
+                    verticalAlignment: Text.AlignVCenter
+                    elide: Text.ElideRight
+                }
+            }
+            MouseArea {
+                id: cmiArea
+                anchors.fill: parent
+                hoverEnabled: true
+                cursorShape: Qt.PointingHandCursor
+                onClicked: { cmRoot.close(); cmRoot.picked(modelData.action) }
+            }
+        }
+    }
+}