Typography polish: tighten GitHub-like font rendering and spacing

[justin808]

Context

Current docs styling is much closer to GitHub now, but typography still feels slightly off (font rendering/weight/spacing in nav and content).

Relevant file: prototypes/docusaurus/src/css/custom.css

Goal

Do a focused typography polish pass for a cleaner GitHub-like reading experience.

Scope

• Tune font metrics and weight across navbar, sidebar, body, and code.
• Review line-height and spacing rhythm for sidebar lists and long-form content.
• Validate in Chrome/Safari on macOS and mobile widths.

Acceptance criteria

• Typography feels consistent between top nav, sidebar, and markdown content.
• No regressions in dark mode readability.
• Before/after screenshots for at least one docs page and one marketing page.