Fix Frontend testing documentation drift

[Ekhorn]

Update the frontend testing guide

The guide had drifted from ui/ since it was last refreshed, and still documented the npmTestUI Gradle task removed in openremote/openremote#3230.

Changelog

• Rewrite "Running the test" around npmTest and -p scoping, drop the removed npmTestUI task, and document -Pargs for sharding app tests
• Split the Playwright config step into the app and component factories (defineAppConfig / defineCtConfig), replacing the stale default import
• Add the app variant of the npmTest Gradle task, which installs only the test workspaces and forwards Playwright arguments
• Correct ui/test/app.config.ts to ui/test/app.config.cts
• Describe the Shared (apps) and CtShared (components) fixture split
• Use fixtures that exist in the app test boilerplate
• Drop the no-op worker: 1 from the setup/teardown snippet
• Add newlines around admonition contents

The worker change is tracked in openremote/openremote#3251 ui/fix-incorrect-worker-setting, so this should land after that fix.

[Ekhorn]

Decided that we'll get rid of the task entirely. Will update the PR once openremote/openremote#3230 is ready.