refactor(renderers): satori migration — Phase 2: renderErrorCard

[evirunurm]

Part of #44

Simplest renderer — no dynamic data, no icons, no variants. Migrate first as a smoke test for the satori setup.

Tasks

• Create scripts/renderers/ErrorCard.tsx as a pure JSX component
• Replace renderErrorCard.ts body with a satori(...) call wrapping the new component
• Make the function async, update the call site in api/stats.ts
• Update renderErrorCard.test.ts to use await

Depends on

• Phase 1 (refactor(renderers): migrate SVG generation to satori #44)