Menu popup: visibility, zoom sync, and tab fixes

Track menu popup visibility and propagate zoom level to the popup. Add menu_popup_visible_ flag, SendMenuPopupZoom(), and call it when creating/showing the popup and when adjusting zoom. Make CreateNewTab accept an optional URL and route popup/new-tab flows to it. Prevent per-tab child closes from triggering app shutdown by tracking closing_tab_browsers_ and adding ForgetClosingTabBrowser(). Treat MenuPopup role specially when enabling frame hit-testing.

Platform: remove usage of ApplyRoundedBrowserRegion and switch menu popup sizing to use resized client_size helpers (consolidate calculations across Win/Mac/Linux). UI: update menu-popup CSS/JS (new styling, font, zoom formatting API via NebulaMenuPopup.setZoomLevel), wire settings to use nebulaNative.postMessage for new-tab, and remove the static menu-popup.html. Misc: small chrome CSS/JS tweaks and ensure chrome state includes zoomLevel.

src/platform/mac/browser_host_mac.cpp

@@ -43,26 +43,21 @@ std::pair<int, int> ParentClientSize(NativeWindow parent) {
 }
 
 Rect MenuPopupRect(NativeWindow parent, const BrowserLayout& layout) {
-    const auto [client_right, client_bottom] = ParentClientSize(parent);
+    const auto client_size = ParentClientSize(parent);
     const int width = 260;
     const int height = 258;
     const int margin = 12;
     const int overlap = 2;
-    const int x = std::max(0, client_right - width - margin);
+    const int x = std::max(0, client_size.first - width - margin);
     const int y = std::max(0, layout.chrome.y + layout.chrome.height - overlap);
     return {
         x,
         y,
-        std::min(client_right, x + width) - x,
-        std::min(client_bottom, y + height) - y,
+        std::min(client_size.first, x + width) - x,
+        std::min(client_size.second, y + height) - y,
     };
 }
 
-void ApplyRoundedBrowserRegion(NativeWindow browser_window, int corner_radius) {
-    UNREFERENCED_PARAMETER(browser_window);
-    UNREFERENCED_PARAMETER(corner_radius);
-}
-
 std::string CacheBusterToken() {
     return "0";
 }