docs(website): add enterprise use-cases section and fix nav wrapping

[babltiga]

What

Adds a Use cases section (#use-cases) to the marketing homepage, between "How it works" and "Install", and fixes the header navigation so items never wrap onto two lines at any screen size.

Use-cases section

• Six enterprise use cases as alternating narrative rows, each with a persona eyebrow tag, problem→solution copy, three benefit bullets, and a CSS-only mock visual (reusing the existing .mock / .pill patterns, theme variables only):
  1. Production access without shared credentials — platform & infrastructure teams
  2. Just-in-time access & break-glass — SRE & on-call teams
  3. AI pre-screened review at scale — engineering managers & DBAs
  4. Audit readiness (SOC 2 / ISO 27001 / GDPR) — compliance & security teams
  5. Data privacy operations (masking, retention, right-to-erasure) — data protection teams
  6. API access governance (REST/SOAP/GraphQL/gRPC) — integration & backend teams
• Every claim maps to a shipped capability; content sources recorded in the website/README.md source map (docs/02, docs/05, docs/07, docs/17).
• Section linked from the desktop nav, mobile nav, and footer Product column.

Nav fix

Adding a seventh link surfaced that nav items wrapped internally ("How it works", the GitHub chip) because they lacked white-space: nowrap — and with nowrap, the full menu was ~40px wider than the max content width. Fixed by:

• nowrap on nav links, GitHub chip, and mobile-panel links; tightened nav spacing so the full menu genuinely fits.
• Progressive collapse ladder: ≤1280px drops the duplicate "Quick start", ≤1140px the GitHub chip, ≤1024px folds links into the hamburger (was 720px), ≤520px drops the nav CTA (hero CTA remains).
• Hamburger moved to the far right (order), fixing a pre-existing bug where its dropdown opened partially off-screen.

Why

The homepage pitched features and mechanics but never showed when/why an enterprise would adopt AccessFlow — visitors had no problem-centric entry point. Copy is written at a mid-technical level for engineering leaders and security/compliance stakeholders.

Reviewer notes

• Static site, no build step; both pages share styles.css, so the nav fix applies to docs/index.html too.
• Verified via Playwright: HTML tag-balance parse; a 16-width sweep (320–1680px, both pages) asserting every visible nav item is single-line and on-screen (incl. the opened hamburger panel); section screenshots in dark/light/mobile — no horizontal overflow from the new section.
• A pre-existing mobile overflow (hero terminal, install tabs; ~211px at 390px wide, identical on baseline) is intentionally not addressed here — tracked separately.