fix(e2e): add paginated mock for /api/invoices and remove fragile networkidle wait

- Return { data: [], total: 0 } for /api/invoices instead of falling through to generic []
- Removes waitForLoadState('networkidle') from admin invoices test (fragile in CI)
- Fixes GRO-558

apps/e2e/tests/navigation.spec.ts

@@ -85,7 +85,6 @@ 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();
 });