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/QbzTabBar.qml

@@ -0,0 +1,102 @@
+// QbzTabBar — the segmented tab pill bar (primitives/SegmentedTabBar
+// .slint), consolidated in phase 22 from HomeView's discover tab pill and
+// LibraryView's SegTab row (+ the playlists sub-tab via `compact`).
+// Arms: `tabs` ([{id, label, count?}]), `activeId`, `counts` (11px count
+// chip), `underline` (2px accent active cue — the Slint redundant-shape
+// cue), `compact` (24px mini variant). Emits selected(id); the host owns
+// side effects (scroll resets, refetches).
+// Deliberately NOT absorbed (Slint keeps them per-surface too): the
+// SearchView underline tab strip, QueuePanel's text tabs, ArtistView's
+// scroll-spy jump links.
+
+import QtQuick
+import com.blitzfc.qbz
+import "../theme"
+
+Rectangle {
+    id: root
+
+    property var tabs: []
+    property string activeId: ""
+    property bool counts: false
+    property bool underline: false
+    property bool compact: false
+    signal selected(string id)
+
+    QbzTheme { id: theme }
+
+    width: segBarRow.width
+    height: segBarRow.height
+    color: theme.surfaceElevated
+    radius: 6
+
+    Row {
+        id: segBarRow
+        padding: root.compact ? 2 : 3
+        spacing: 4
+        Repeater {
+            model: root.tabs
+            delegate: Rectangle {
+                id: segRoot
+                required property var modelData
+                readonly property bool active: modelData.id === root.activeId
+                readonly property int count: modelData.count || 0
+
+                width: segRow.implicitWidth
+                height: segRow.implicitHeight
+                radius: 4
+                color: active ? theme.surfaceMain
+                     : segArea.containsMouse ? theme.surfaceHover : "transparent"
+
+                Row {
+                    id: segRow
+                    leftPadding: root.compact ? 8 : 12
+                    rightPadding: (root.compact ? 8 : 12) - (root.counts && segRoot.count > 0 ? 4 : 0)
+                    topPadding: root.compact ? 4 : 6
+                    bottomPadding: root.compact ? 4 : 6
+                    spacing: 7
+                    Text {
+                        text: segRoot.modelData.label
+                        color: segRoot.active ? theme.textPrimary : theme.textMuted
+                        font.pixelSize: root.compact ? 12 : theme.fontLegal
+                        font.weight: theme.weightMedium
+                        anchors.verticalCenter: parent.verticalCenter
+                    }
+                    Rectangle {
+                        visible: root.counts && segRoot.count > 0
+                        width: Math.max(18, countText.implicitWidth + 10)
+                        height: 16
+                        radius: 8
+                        anchors.verticalCenter: parent.verticalCenter
+                        color: segRoot.active ? "#26ffffff" : "#14ffffff"
+                        Text {
+                            id: countText
+                            anchors.centerIn: parent
+                            text: segRoot.count
+                            color: segRoot.active ? theme.textPrimary : theme.textSecondary
+                            font.pixelSize: 11
+                            font.weight: theme.weightMedium
+                        }
+                    }
+                }
+                // Active underline (redundant shape cue, Slint Segment).
+                Rectangle {
+                    visible: root.underline && segRoot.active
+                    x: 4
+                    width: parent.width - 8
+                    height: 2
+                    y: parent.height - 2
+                    radius: 1
+                    color: theme.accent
+                }
+                MouseArea {
+                    id: segArea
+                    anchors.fill: parent
+                    hoverEnabled: true
+                    cursorShape: Qt.PointingHandCursor
+                    onClicked: root.selected(segRoot.modelData.id)
+                }
+            }
+        }
+    }
+}