fix(window): force dark native title bar

Apply Electron nativeTheme dark mode before creating the main window so the Windows title bar remains dark across local, server, and RDP user profiles. Add focused coverage for the native theme contract.

tests/native-theme.test.ts

@@ -0,0 +1,12 @@
+import { describe, expect, it } from "vitest";
+import { forceDarkNativeTheme } from "../src/main/native-theme";
+
+describe("native window theme", () => {
+  it("forces the native Windows title bar to use the dark theme", () => {
+    const theme = { themeSource: "system" };
+
+    forceDarkNativeTheme(theme);
+
+    expect(theme.themeSource).toBe("dark");
+  });
+});