feat(titlebar): theme-colored window bar on macOS (#1199)

* feat(titlebar): theme-colored window bar on macOS

macOS showed the grey native title bar that ignores the app theme
(#1198). Enable titleBarStyle: Overlay so the webview reaches the top
edge and the native traffic lights float over the existing in-page
title bar, which already paints with the active theme.

Reuses the Linux title-bar grid: the bar is shown on macOS too (hidden
in native fullscreen), without the custom window buttons since the
traffic lights stay native. The GTK resize grips are now Linux-only.
Renamed the stylesheet to drop the misleading linux-only name.

* docs(changelog): macOS themed title bar

* fix(titlebar): crisp macOS bar text — drop transform layer and shadow

* fix(titlebar): drop now-playing text from the macOS bar

src-tauri/tauri.conf.json

@@ -22,6 +22,7 @@
         "fullscreen": false,
         "decorations": true,
         "transparent": false,
+        "titleBarStyle": "Overlay",
         "visible": false,
         "backgroundColor": "#1e1e2e",
         "dragDropEnabled": false