feat(ui-lab): add first-run and update scenarios

Add separate UI Lab scenarios for true first-run onboarding and post-update What's New checks. Keep empty-library fixtures reusable across mock data, tagger readiness, worker pause defaults, and launch-time version state so browser verification can exercise the real flows.

src/dev/mockScenarios.ts

@@ -9,6 +9,8 @@ export type MockScenario =
   | 'extreme'
   | 'unready'
   | 'joytag-unready'
+  | 'new-user'
+  | 'just-updated'
 
 const SCENARIOS = new Set<MockScenario>([
   'rich',
@@ -21,8 +23,17 @@ const SCENARIOS = new Set<MockScenario>([
   'extreme',
   'unready',
   'joytag-unready',
+  'new-user',
+  'just-updated',
 ])
 
+// Scenarios that start with no folders or media. 'empty' is a bare library
+// with onboarding already behind it; 'new-user' is the true first run and
+// additionally opens the onboarding tour with no tagger model downloaded.
+export function isEmptyLibraryScenario(scenario: MockScenario): boolean {
+  return scenario === 'empty' || scenario === 'new-user'
+}
+
 export function getMockScenario(): MockScenario {
   if (typeof window === 'undefined') return 'rich'
   const value = new URLSearchParams(window.location.search).get('scenario')