feat: hub nav strip below main header

[Flotapponnier]

Summary

• Adds a slim secondary navigation strip (h-8, font-mono 11px uppercase) below the sticky main header
• Shows 10 hub pages not in the primary nav: Chains, Prediction markets, RPC, Data APIs, Perpetuals, Bridge, Trading apps, Compare, Alternatives, Answers
• Active page highlighted with accent underline (same pattern as primary nav)
• Horizontally scrollable on narrow viewports (scrollbar-none)
• Hidden when mobile hamburger menu is open (all hubs already listed there in the expanded menu)

Test plan

• Strip visible on desktop and mobile
• Active item underlined on each hub page
• Scrollable on narrow viewports without visible scrollbar
• Mobile hamburger menu still lists all hub pages