onboarding: cleaner login-import summary and resume fallback copy

- render_markdown now groups results under 'Logins imported', listing
  reused logins and any skipped sources with counts, instead of a flat
  'Imported N source(s)' dump.
- Per-source lines read 'Provider (from Source): detail'.
- Resume fallback message no longer says 'Couldn't open ... directly';
  it explains it will search for the most recent session and continue.