Remove Cost tab and add in-app PDF/document viewer

- Remove the Cost tab from all job detail views (job_details_full_view,
  job_tabs_full_screen, job_all_tabs_full_view)
- Add InAppDocViewer page that renders PDFs natively with pinch-to-zoom
  using flutter_pdfview (page swiping, page counter in app bar)
- Documents now open in-app instead of launching external apps: PDFs
  render inline, Word/other files fall back to system viewer
- No new dependencies needed (flutter_pdfview already in pubspec.yaml)

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>
