Serve the root page for the plane this instance is running as.

P2 change: Owner plane (:8795, ROUTER_REQUIRE_KEY off) now redirects to /console (the Liminal two-versions consolidated app) with a 301. Sell plane (:8796, ROUTER_REQUIRE_KEY=1) serves static/landing.html — pricing + quickstart with the live pay link ({{PAY_URL}}, same resolution chain as the 402) and this request's own external base URL ({{BASE_URL}}, honoring X-Forwarded-Proto from cloudflared) substituted at serve time. Paths resolved relative to this module (NOT cwd); each falls back so the route never 500s.