test: add Playwright end-to-end example for FormView (#16)

Adds the first browser-driven test to the project: PlaywrightTestBase starts
the app on a random port and drives a headless Chromium against it, and
FormViewPlaywrightTest covers the form's rendering, binder validation and the
save/reset buttons.

The tests skip themselves via a JUnit assumption when no browser can be
launched, so runners without Playwright browsers stay green.

Closes #16

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01Ue9ZtWUBQF4SuSHpzZ3zwq

CLAUDE.md

@@ -9,9 +9,14 @@ This file provides guidance to Claude Code (claude.ai/code) when working with co
 ./mvnw compile              # compile only
 ./mvnw package               # production build → target/*.jar
 java -jar target/*.jar       # run production jar
+./mvnw test                  # run all tests
 ```
 
-No test sources exist yet (`src/test` is empty) — `spring-boot-starter-test` and `browserless-test-spring` are on the classpath but unused.
+**Tests** live in two flavours, both run by surefire (`*Test`):
+- `views/*Test` — browserless (`browserless-test-spring`): the Vaadin UI is built server-side and asserted on the component tree, no browser. Fast (seconds).
+- `e2e/*PlaywrightTest` — real headless Chromium (`com.microsoft.playwright:playwright`) against the app on a random port, via `e2e/PlaywrightTestBase`. Slow (~1 min). Playwright downloads its browsers into `~/.cache/ms-playwright` on first run; if that fails (offline runner, missing system libs) the tests are **skipped** via a JUnit assumption instead of failing the build.
+
+E2E assertions match the **English** captions: German is the unsuffixed fallback bundle (`translations.properties`), so `de` is not a provided locale and Vaadin serves `translations_en.properties` for a German browser. `PlaywrightTestBase` pins the context locale to `en-US` to make that explicit.
 
 **Java 25 toolchain required** (`pom.xml` sets `java.version=25`). If the default JDK on PATH is older (check `java -version`), point `JAVA_HOME` at a Java 25 install for the Maven build, e.g.:
 ```bash
@@ -43,7 +48,7 @@ UI copy/data (chart labels, notifications) is in German.
 ## AI-Autofix – Regeln
 - Nur Issues mit Label `ai-ready` bearbeiten.
 - Branch-Namensschema: `ai/issue-<nr>-<kurz-slug>`.
-- Kein Fix ohne grüne Tests. Test-Befehl: `<dein Testkommando>`.
+- Kein Fix ohne grüne Tests. Test-Befehl: `./mvnw test`.
 - Bei Unklarheit / Scope > ~200 Zeilen Diff: NICHT fixen,
   stattdessen Issue kommentieren ("needs human") und Label `ai-blocked` setzen.
 - PR-Beschreibung MUSS `Closes #<nr>` enthalten.

src/test/java/com/example/e2e/FormViewPlaywrightTest.java

@@ -0,0 +1,86 @@
+package com.example.e2e;
+
+import com.microsoft.playwright.Locator;
+import com.microsoft.playwright.Page;
+import com.microsoft.playwright.options.AriaRole;
+import org.junit.jupiter.api.BeforeEach;
+import org.junit.jupiter.api.Test;
+
+import static com.microsoft.playwright.assertions.PlaywrightAssertions.assertThat;
+
+/**
+ * End-to-end example covering {@link com.example.views.FormView}: rendering,
+ * binder validation and the two form buttons. Captions are English — see
+ * {@link PlaywrightTestBase#openPage()} for why.
+ */
+class FormViewPlaywrightTest extends PlaywrightTestBase {
+
+    @BeforeEach
+    void openFormView() {
+        navigate("formular");
+    }
+
+    @Test
+    void form_rendersItsFieldsAndTheReadOnlyAge() {
+        assertThat(page.getByText("Registration")).isVisible();
+        assertThat(field("First name")).isVisible();
+        assertThat(field("Last name")).isVisible();
+        assertThat(field("Email")).isVisible();
+        // preset on the bean and bound read-only
+        assertThat(field("Age")).hasValue("37");
+        assertThat(field("Age")).not().isEditable();
+    }
+
+    @Test
+    void save_withoutRequiredFields_showsErrorNotification() {
+        button("Save").click();
+
+        assertThat(notification()).containsText("Please check your entries");
+    }
+
+    @Test
+    void save_withInvalidEmail_showsErrorNotification() {
+        field("First name").fill("Max");
+        field("Last name").fill("Mustermann");
+        field("Email").fill("not-an-email");
+
+        button("Save").click();
+
+        assertThat(notification()).containsText("Please check your entries");
+    }
+
+    @Test
+    void save_withValidInput_showsSuccessNotification() {
+        field("First name").fill("Max");
+        field("Last name").fill("Mustermann");
+        field("Email").fill("max@example.com");
+
+        button("Save").click();
+
+        assertThat(notification()).containsText("Saved: Max Mustermann");
+    }
+
+    @Test
+    void reset_clearsTheEnteredValues() {
+        field("First name").fill("Max");
+        field("Last name").fill("Mustermann");
+
+        button("Reset").click();
+
+        assertThat(field("First name")).hasValue("");
+        assertThat(field("Last name")).hasValue("");
+    }
+
+    /** The input inside the Vaadin field carrying {@code label}. */
+    private Locator field(String label) {
+        return page.getByLabel(label, new Page.GetByLabelOptions().setExact(true));
+    }
+
+    private Locator button(String caption) {
+        return page.getByRole(AriaRole.BUTTON, new Page.GetByRoleOptions().setName(caption).setExact(true));
+    }
+
+    private Locator notification() {
+        return page.locator("vaadin-notification-card");
+    }
+}

src/test/java/com/example/e2e/PlaywrightTestBase.java

@@ -0,0 +1,93 @@
+package com.example.e2e;
+
+import com.example.Application;
+import com.microsoft.playwright.Browser;
+import com.microsoft.playwright.BrowserContext;
+import com.microsoft.playwright.BrowserType;
+import com.microsoft.playwright.Page;
+import com.microsoft.playwright.Playwright;
+import com.microsoft.playwright.assertions.PlaywrightAssertions;
+import org.junit.jupiter.api.AfterAll;
+import org.junit.jupiter.api.AfterEach;
+import org.junit.jupiter.api.Assumptions;
+import org.junit.jupiter.api.BeforeAll;
+import org.junit.jupiter.api.BeforeEach;
+import org.springframework.boot.test.context.SpringBootTest;
+import org.springframework.boot.test.web.server.LocalServerPort;
+
+import java.util.Locale;
+
+/**
+ * Base class for end-to-end tests that drive the running application in a real
+ * browser. The Spring Boot app is started on a random port, Playwright opens a
+ * headless Chromium against it, and every test gets a fresh browser context —
+ * so each test also gets a fresh Vaadin session.
+ *
+ * <p>Playwright downloads its browsers into {@code ~/.cache/ms-playwright} on
+ * first use. If that is not possible (offline runner, missing system libraries)
+ * the tests are <em>skipped</em> rather than failed, so the build stays green
+ * on runners that cannot host a browser.
+ */
+@SpringBootTest(classes = Application.class, webEnvironment = SpringBootTest.WebEnvironment.RANDOM_PORT)
+public abstract class PlaywrightTestBase {
+
+    /** Generous: the first request in dev mode has to bootstrap the frontend. */
+    private static final double TIMEOUT_MS = 60_000;
+
+    private static Playwright playwright;
+    private static Browser browser;
+
+    @LocalServerPort
+    private int port;
+
+    private BrowserContext context;
+    protected Page page;
+
+    @BeforeAll
+    static void launchBrowser() {
+        try {
+            playwright = Playwright.create();
+            browser = playwright.chromium().launch(new BrowserType.LaunchOptions().setHeadless(true));
+        } catch (Exception | UnsatisfiedLinkError e) {
+            closeBrowser();
+            Assumptions.abort("no Playwright browser available: " + e.getMessage());
+        }
+        PlaywrightAssertions.setDefaultAssertionTimeout(TIMEOUT_MS);
+    }
+
+    @AfterAll
+    static void closeBrowser() {
+        if (browser != null) {
+            browser.close();
+            browser = null;
+        }
+        if (playwright != null) {
+            playwright.close();
+            playwright = null;
+        }
+    }
+
+    @BeforeEach
+    void openPage() {
+        // Pin the locale so the assertions have one fixed set of captions to
+        // match. English, not German: the German bundle is the unsuffixed
+        // fallback (`translations.properties`), so `de` is not a provided
+        // locale and Vaadin serves `translations_en.properties` for it anyway.
+        context = browser.newContext(new Browser.NewContextOptions().setLocale(Locale.US.toLanguageTag()));
+        context.setDefaultTimeout(TIMEOUT_MS);
+        page = context.newPage();
+    }
+
+    @AfterEach
+    void closePage() {
+        if (context != null) {
+            context.close();
+            context = null;
+        }
+    }
+
+    /** Opens a Vaadin route, e.g. {@code navigate("formular")}. */
+    protected void navigate(String route) {
+        page.navigate("http://localhost:" + port + "/" + route);
+    }
+}