fix(e2e): use domcontentloaded instead of networkidle in admin invoices test

The networkidle wait causes flakiness in CI due to slow external resource loading.
Use domcontentloaded which fires earlier and is sufficient for SPA navigation checks.

Co-authored-by: Pawla Abdul (Bot) <pawla@groombook.dev>
Co-authored-by: Paperclip <noreply@paperclip.ing>

apps/e2e/tests/navigation.spec.ts

@@ -85,6 +85,7 @@ test("admin staff page loads", async ({ page }) => {
 
 test("admin invoices page loads", async ({ page }) => {
   await page.goto("/admin/invoices");
+  await page.waitForLoadState("domcontentloaded");
   await expect(page.getByText("GroomBook")).toBeVisible();
   await expect(page.getByRole("link", { name: "Invoices" })).toBeVisible();
 });