feat: PromptArch v2.1 - Agentic AI Assist 3-Step Flow + Slides Enhancements
- Implemented Plan -> Approve -> Generate workflow in AIAssist
- Added Plan Review Card with architecture/tech stack preview
- Added Reveal.js Markdown export to SlidesGenerator
- Created /api/ai-assist and /api/slides orchestration routes
- Fixed all syntax errors and build issues
- Maintained premium blue/teal glassmorphism design
