feat: separate create/edit form pages, Playwright test infrastructure, /auth/token endpoint

Move all CRUD create/edit forms from inline on list pages to dedicated form pages
with back-button navigation and post-save redirect. Add Playwright Docker container
(browser server on port 3000) with 25 passing E2E tests covering login, navigation,
and site CRUD workflows. Add POST /auth/token endpoint for clean JWT retrieval.

tests/ScadaLink.CentralUI.PlaywrightTests/xunit.runner.json

@@ -0,0 +1,4 @@
+{
+  "$schema": "https://xunit.net/schema/current/xunit.runner.schema.json",
+  "maxParallelThreads": 1
+}