chore: document and stabilize vitest browser setup

tests/browser/example.browser.test.ts

@@ -0,0 +1,40 @@
+import { expect, test } from "vitest"
+
+test("CTA button snapshot", async () => {
+  const html = `
+    <main
+      style="
+        font-family: 'Inter', sans-serif;
+        padding: 48px;
+        display: flex;
+        align-items: center;
+        justify-content: center;
+        background: linear-gradient(135deg, #111827, #1f2937);
+        min-height: 320px;
+      "
+    >
+      <button
+        data-testid="cta"
+        style="
+          font-size: 18px;
+          padding: 14px 28px;
+          border-radius: 9999px;
+          border: none;
+          color: white;
+          background: #2563eb;
+          box-shadow: 0 10px 20px rgba(37, 99, 235, 0.35);
+          cursor: pointer;
+        "
+      >
+        Abrir chamado
+      </button>
+    </main>
+  `
+
+  document.body.innerHTML = html
+
+  const ctaButton = document.querySelector("[data-testid='cta']")
+  expect(ctaButton).toBeTruthy()
+
+  await expect(document.body).toMatchScreenshot("cta-button")
+})