macOS: Update core surface size when config changes (#9525)

More or less the same as #9523, but for config reloads. Matters if
`scrollbar = never` has been added or removed since the last config
reload.

macos/Sources/Ghostty/SurfaceScrollView.swift

@@ -129,7 +129,7 @@ class SurfaceScrollView: NSView {
         surfaceView.$derivedConfig
             .sink { [weak self] _ in
                 DispatchQueue.main.async { [weak self] in
-                    self?.synchronizeAppearance()
+                    self?.handleConfigChange()
                 }
             }
             .store(in: &cancellables)
@@ -232,6 +232,12 @@ class SurfaceScrollView: NSView {
     private func handleScrollerStyleChange() {
         synchronizeCoreSurface()
     }
+
+    /// Handles config changes
+    private func handleConfigChange() {
+        synchronizeAppearance()
+        synchronizeCoreSurface()
+    }
     
     /// Handles live scroll events (user actively dragging the scrollbar).
     ///