fix: use light background color for Electron window chrome

Change BrowserWindow backgroundColor from dark #13110f (leftover from
old design tokens) to #ffffff so the title bar area matches light mode.
The old dark color was visible in light mode as a dark title bar strip.

src/main/desktop-shell-service.ts

@@ -206,7 +206,7 @@ export function createDesktopShellService({
       y: savedWindowState.bounds.y,
       minWidth: 1180,
       minHeight: 760,
-      backgroundColor: "#13110f",
+      backgroundColor: "#ffffff",
       titleBarStyle: process.platform === "darwin" ? "hiddenInset" : "default",
       webPreferences: {
         preload: preloadPath,