fix(settings): harden migration and accessibility

Apply interface scaling through Qt at startup, complete keyboard navigation, move account migration into setup and progress modals, and defer Subsonic sync while the server scans.

cx

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

@@ -149,8 +149,9 @@ Rectangle {
     // Escape stack) lives in Rust behind QbzHotkeys.keyPressed; the QML side
     // only computes the gate (§1.4.4). Null activeFocusItem passes the gate
     // (the semantically right case). Every existing Keys. handler accepts
-    // only what it owns, and all buttons are MouseArea-based and can never
-    // take keyboard focus — verified non-conflicts (§4.1).
+    // only what it owns. Shared controls accept their Space/Enter events
+    // before they bubble here; in particular a focused Settings toggle owns
+    // Space, so toggling it cannot also trigger global play/pause (§4.1).
     Keys.onPressed: function (event) {
         var w = root.Window.window
         var afi = w !== null ? w.activeFocusItem : null