feat(landing): add Get in touch contact section + nav CTAs

[Markeljan]

Summary

• New Enterprise section sits below Install and renders a minimal contact card. Submitting it opens a Gmail compose draft to mark@semiotic.ai in a new tab.
• Message field is optional; an empty submit prefills an "I'm interested in agentsec, could you tell me more?" body so anyone can fire off a 1-click intro.
• Hero secondary CTA, Header nav + button, and Footer links all point at #enterprise so the form is reachable from any vantage point. Install copy and order otherwise unchanged.

Test plan

• bun run check and bun run lint pass
• Visit the landing page, click "Get in touch" in the hero / header → page anchors to the form section
• Submit the form with the message field empty → Gmail compose opens with the default "I'm interested" body, to=mark@semiotic.ai, subject agentsec — inquiry
• Submit with a typed message → Gmail compose opens with that body verbatim
• If popup is blocked, the inline "Didn't open? Click here" link opens the same URL
• Mobile layout: form stacks correctly at 375px wide

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