refactor: trim onboarding intro flow and About tab

[quiet-node]

Description

Thuki moves to maintenance mode. The README says so up front now, and the onboarding intro flow and the Settings About tab are trimmed to match.

Key changes

• README: adds a Project status section stating that the app is stable and maintained but no longer under active feature development. Removes the Roadmap and Founder note sections, drops a stale "coming soon" provider bullet whose anchor no longer exists, and updates the title and status badge.
• Onboarding: the intro stage renders the tips card directly instead of running a two-step sequence. The backend already covers the panel on the transition into intro, and the fit hook reveals it once the card settles, so the frontend swap that existed only to bridge those two cards is no longer needed.
• Settings: the About tab drops a secondary card, along with the backend command and the two constants that only that card used. The docs section documenting those constants goes with them.
• Removes a DownloadStatusStrip variant whose only consumer was the deleted onboarding step.

Testing

bun run test:all:coverage and bun run validate-build both pass. Coverage holds at 100% on both frontend and backend.