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/QbzSlider.qml

@@ -0,0 +1,75 @@
+// QbzSlider (primitives/QbzSlider.slint) — extracted from SettingsView.qml
+// in phase 19. 200x22, 4px r2 track, accent fill, 16px thumb; integer
+// steps. Like the Slint original the thumb follows the pointer during a
+// drag (local dragValue) and commits each step via changed(int).
+
+import QtQuick
+import com.blitzfc.qbz
+import "../theme"
+
+Item {
+    property int minimum: 0
+    property int maximum: 10
+    property int value: 0
+    signal changed(int newValue)
+
+    QbzTheme { id: theme }
+
+    width: 200
+    height: 22
+
+    readonly property int thumbSize: 16
+    readonly property real travel: width - thumbSize
+    readonly property real fraction: maximum > minimum
+        ? Math.max(0, Math.min(1, (value - minimum) / (maximum - minimum))) : 0
+    property bool dragging: false
+    property real dragFraction: fraction
+    readonly property real shownFraction: dragging ? dragFraction : fraction
+    onFractionChanged: if (!dragging) dragFraction = fraction
+
+    function commit(frac) {
+        const v = Math.round(minimum + Math.max(0, Math.min(1, frac)) * (maximum - minimum))
+        if (v !== value) changed(v)
+    }
+
+    Rectangle { // track
+        x: 0
+        y: Math.round((parent.height - height) / 2)
+        width: parent.width
+        height: 4
+        radius: 2
+        color: theme.surfaceElevated
+    }
+    Rectangle { // accent fill
+        x: 0
+        y: Math.round((parent.height - height) / 2)
+        width: parent.thumbSize / 2 + parent.shownFraction * parent.travel
+        height: 4
+        radius: 2
+        color: theme.accent
+    }
+    Rectangle { // thumb
+        width: parent.thumbSize
+        height: parent.thumbSize
+        radius: parent.thumbSize / 2
+        x: parent.shownFraction * parent.travel
+        anchors.verticalCenter: parent.verticalCenter
+        color: theme.textPrimary
+    }
+    MouseArea {
+        anchors.fill: parent
+        cursorShape: Qt.PointingHandCursor
+        onPressed: {
+            parent.dragging = true
+            parent.dragFraction = Math.max(0, Math.min(1, (mouse.x - parent.thumbSize / 2) / parent.travel))
+            parent.commit(parent.dragFraction)
+        }
+        onPositionChanged: {
+            if (pressed) {
+                parent.dragFraction = Math.max(0, Math.min(1, (mouse.x - parent.thumbSize / 2) / parent.travel))
+                parent.commit(parent.dragFraction)
+            }
+        }
+        onReleased: parent.dragging = false
+    }
+}