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/SettingsSpacer.qml

@@ -0,0 +1,5 @@
+// Settings 12px group spacer — extracted from SettingsView.qml in phase 19.
+
+import QtQuick
+
+Item { width: 1; height: 12 }