Thread view parity, PDF overhaul, export menu and server fixes

- Thread detail view now mirrors consensus view using PhaseCard
  (PROPOSE/CHALLENGE/REVISE) instead of flat TurnCard contributions
- ThreadNav upgraded to phase-level navigation with model names
- PDF export: styled cover page with duh. branding and quoted question,
  Contents page with dot leaders and clickable page links,
  colored section header bars, phase-grouped contributions,
  consolidated Sources section with numbered clickable URLs
- ExportMenu dropdown: opaque background, fixed hover state token
- Thread header: opaque background + z-index for dropdown visibility
- Server startup: create_all for fresh file-based SQLite databases
- ensure_schema: gracefully skip missing tables

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: msitarzewski