feat: integrate welcome screen with tests and fix App tests
WelcomeScreen component (14 tests):
- welcome mode: title, buttons, hint, loading, error states
- vault-missing mode: path badge, different button labels
useOnboarding hook (10 tests):
- vault exists → ready, vault missing → welcome
- dismissed + missing → vault-missing
- create vault, open folder, dismiss transitions
- error handling, picker cancellation, command failure fallback
App.test.tsx updated to mock new Tauri commands
(check_vault_exists, get_default_vault_path).
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>
2aef46709d