fix(qt): keep the Qt 6.9 SafeArea/padding API macOS-only

Window.topPadding..rightPadding and the SafeArea attached type are Qt
6.9; on the Linux pin (6.8 LTS) declaring them on the ApplicationWindow
refuses the whole component at Main.qml:173 ("Cannot assign to
non-existent property"), measured on the CI VM 2026-08-25 and on a Pi 4
with Debian 13's Qt 6.8.2. The paddings are now set imperatively from a
Loader active only on macOS, and HeaderBar reads SafeArea.margins.left
through a macOS-only Loader into a plain property, so no binding names
the type on other platforms. Behaviour on macOS is unchanged.

cc

crates/qbz-qt/qml/Main.qml

@@ -167,10 +167,24 @@ ApplicationWindow {
     // takes the safe area over from Qt entirely. `HeaderBar.chromeLeftInset`
     // is where that responsibility is discharged: it reads
     // `SafeArea.margins.left` to clear the traffic lights.
-    topPadding: 0
-    bottomPadding: 0
-    leftPadding: 0
-    rightPadding: 0
+    // The four Window paddings are Qt 6.9 API (SafeArea landed in 6.9). The
+    // Linux pin is Qt 6.8 LTS, where declaring them here refuses the whole
+    // component ("Cannot assign to non-existent property") — measured on the
+    // CI VM, 2026-08-25. They only matter on macOS, so they are set there,
+    // imperatively, from a Loader that never instantiates elsewhere.
+    Loader {
+        active: QbzShell.isMacos
+        sourceComponent: Component {
+            Item {
+                Component.onCompleted: {
+                    window.topPadding = 0
+                    window.bottomPadding = 0
+                    window.leftPadding = 0
+                    window.rightPadding = 0
+                }
+            }
+        }
+    }
     color: "#1a1a1a"
 
     // Phase 23: every domain singleton boots (registers its Qt-thread