Add Deep Dive landing page

[Aprasaks]

Summary

• Add a new static /deep-dive landing page
• Keep the Archive-style ivory editorial tone and SiteHeader
• Add a left sidebar with Deep Dive description, keywords, and AI Librarian card
• Add a featured Deep Dive cover card and static All Deep Dives grid
• Point the Header Deep Dive navigation item to /deep-dive

Verification

• npm run lint -- --max-warnings=0 passed
• npx tsc --noEmit --pretty false passed
• npm run build passed
• Dev route checked at /deep-dive

Notes

• Static UI only; no real Deep Dive data integration yet