Open a fresh draw board (browser redirect)

Human-friendly entry point for sharing a new draw board. Creates a fresh board in draw mode and responds with 302 Found to https://cnvs.app/#<id> — a clean URL you can paste into chat. Optional query seed (mermaid-flowchart / mermaid-mindmap) pre-populates the canvas with an example Mermaid diagram; an absent or unknown seed yields a plain blank draw board.

Shares the POST /api/boards per-IP create cap (5 / 60s per CF-Connecting-IP).