feat(onboarding): wire v2 auth flow, API calls, and completion

- Rewrite import flow: async/await + CSS transition, no setInterval
- Auth first, then import: chooser buttons open auth, onSuccessfulRegistration triggers import
- Add completeAction (CompletedOnboarding, EditTag, ContentTypes) after tags saved
- Persist aiPrompt and signupContext via usePersistentContext (survives OAuth redirect)
- Add logged-in redirect: complete → redirectToApp, incomplete → show chooser
- Use ?step=complete query param to guard extension promo after completion
- Persist extensionSeen flag so promo only shows once
- Remove dead code: topics panel, SELECTABLE_TOPICS, intersection observer, recommendedTopics
- Remove 5-second auto-redirect from feedReadyState
- Fix redirect to use redirectToApp

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: capJavert