onboarding: add telemetry content-sharing consent phase + 60s decision timers

- Add post-import 'Help improve jcode?' consent card (opt-in to sharing
  prompts/transcripts), default No, persisted via telemetry_share_content
  marker (jcode-base telemetry content_sharing_enabled/set_*).
- Give every onboarding decision (login import + telemetry) a 60s countdown
  that auto-selects the highlighted default on timeout.
- Model select now instructs '/model' instead of auto-opening the picker.
- Tests for consent advance/opt-in/y-key and import decision timer.