feat(website): overhaul marketing architecture, typography, blog, and navigation - #30

What

• Redesigned and overhauled the marketing website across all routes (/action, /adapters, /compare, /blog, /docs, and landing sections).
• Standardized typography and monospace discipline across sections.
• Rebuilt navigation with synchronized desktop header, active page breadcrumb indicators, and right-side slide-over mobile drawers mounted via React Portal.
• Updated PRCommentShowcase to support native GitHub Light & Dark mode themes.
• Redesigned blog index with segmented filter capsule bar, minimal cards with upper-right arrow transitions, and unified max-w-6xl article header gradient bleed.

Why

• Elevates the visual identity, brand consistency, and marketing positioning of AgentDiff as a deterministic CI/CD regression gate for AI agents.

How

• Standardized section containers to max-w-6xl.
• Registered YAML prism highlighters in CodeBlock.
• Implemented createPortal for viewport-locked mobile drawers without layout clipping.
• Added scrollbar-gutter: stable to prevent layout shifts.

Testing

• pnpm build in website/ passing with 61/61 static pages generated.
• make lint + uv run python -m pytest 100% green (344 tests passing).

Checklist

• Tested locally on Turbopack dev server and production static build.
• Verified dark/light mode switching.
• Mobile viewport navigation and responsive behavior verified.