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.