Add adapter-node for production deployment

[yousecjoe]

Description

Switch from adapter-auto to adapter-node for production deployment as a standalone Bun/Node server.

• Install @sveltejs/adapter-node
• Update svelte.config.js to use adapter-node
• Configure adapter-node settings (ORIGIN, PROTOCOL_HEADER, HOST_HEADER, etc.) from env.example
• Verify production build with bun run build && bun run preview

Blocked by

• Verify production build #7 (Verify production build)

Parent

Part of #1 (Phase 4)