Adds SignInPrompt (sign in / create account), showSignInPromptProvider,
HomeScreen as ConsumerWidget, l10n de/en, widget/unit tests and a
Playwright web test (#49 criterion 1, QUE-09 criterion 13, QUE-06).
Refs #49
Co-Authored-By: Claude Sonnet 5.5 <noreply@anthropic.com>