feat: serve existing landing/index.html instead of generated HTML

Use include_str! to embed the polished 62KB landing page at compile
time. The existing page has proper design system, animations,
architecture diagrams, competitive comparison, FAQ, and CTA sections.

- GET / now serves landing/index.html (embedded at compile time)
- GET /benchmarks still serves the bar chart benchmarks page
- GET /pricing still serves the 3-tier pricing page
- Both supplementary pages match the existing page's color scheme

Author: nadilas