fix(qt): Windows arm for the gl renderer pref; pin the Basic controls style

The gl/gles/femtovg pref forced QSG_RHI_BACKEND=opengl on every non-macOS
host, dragging Windows off its healthy d3d11 default onto desktop GL. That
is a downgrade, not the GPU tier the pref means, so Windows takes the same
remap macOS already had.

And with no explicit style Qt loads Windows on Windows and Fusion on
Linux, never Basic, so the popups without an explicit background inherit
native chrome. Pinned before QGuiApplication, since it must run before the
first QtQuick.Controls import, and windeployqt then ships one style.

cc

crates/qbz-qt/src/main.rs

@@ -3093,6 +3093,12 @@ fn apply_renderer_preference() {
             if cfg!(target_os = "macos") {
                 log::info!("[renderer] '{choice}' on macOS -> Metal default (the Slint GL remap)");
                 false
+            } else if cfg!(target_os = "windows") {
+                // d3d11 is Qt's healthy default here; desktop GL / ANGLE is a
+                // downgrade, not the GPU tier this pref means. Same remap as
+                // macOS: honour the intent (GPU path), not the literal backend.
+                log::info!("[renderer] '{choice}' on Windows -> d3d11 default (the Slint GL remap)");
+                false
             } else {
                 std::env::set_var("QSG_RHI_BACKEND", "opengl");
                 log::info!("[renderer] '{choice}' -> QSG_RHI_BACKEND=opengl");
@@ -3322,6 +3328,13 @@ fn main() {
     renderer_qt::apply_gpu_preference();
     apply_scroll_physics();
 
+    // D7 (research/02 §4.4): with no explicit style Qt loads `Windows` on
+    // Windows and `Fusion` on Linux — never Basic — and the popups without an
+    // explicit `background:` inherit native chrome. Pin Basic so all three
+    // platforms draw the same thing and windeployqt ships one style. Must run
+    // before the first QtQuick.Controls import, i.e. before the engine exists.
+    cxx_qt_lib::QQuickStyle::set_style(&QString::from("Basic"));
+
     let mut app = QGuiApplication::new();
     let mut engine = QQmlApplicationEngine::new();