Onboarding polish and mobile UX

Improved

• Onboarding "Pick your apps" -- each integration card shows what you get from that app; the CTA reads "Connected apps x/2" until two apps connect, then "See memory in action".
• Mobile chat -- conversations toggle pinned to the top, assistant messages span full width, and the hover-only imports chip is hidden on touch.
• Billing plan cards on mobile -- narrower cards so the next plan peeks in from the edge; slightly lighter frosted-glass tint in dark mode.

Fixed

• Chat no longer yanks you back to the bottom while an assistant reply is streaming -- scroll up to pause auto-follow, scroll back to the bottom to resume.
• Linear sync no longer runs on a schedule when OAuth is disconnected but a team selection is still saved.
• Background imports respect upstream rate-limit backoff more reliably when providers return 429 responses.