Rework the implementation roadmap from parallelizable phases to a strict
sequential order optimized for earliest usable increments. Key changes:
- All 20 user stories in a single sequential queue (no parallelization)
- Progress tracker at the top for status tracking
- US-17 (dark mode) moved from Phase 5 to Increment 4 (before US-15)
- US-14 (PWA) moved to last position
- Deferred AC activation schedule added
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>