Add support for custom providers and local models

@dundalek is attempting to deploy a commit to the Yury Selivanov's projects Team on Vercel.

A member of the Team first needs to authorize it.

I would prefer the custom apiBase approach in this PR to #36. IMO, it is more future-proof.

As a note since creating the PR I discovered draft of llm uri scheme spec.

That would make setting it more convenient, instead of needing 4 env vars like:

LAT_LLM_BASE=https://openrouter.ai/api/v1
LAT_LLM_MODEL=qwen/qwen3-embedding-8b
LAT_LLM_DIMENSIONS=4096
LAT_LLM_KEY=sk-or-v1-xyz

We could use a single var like:

LAT_LLM_KEY=llm://:sk-or-v1-xyz@https://openrouter.ai/api/v1/qwen%2Fqwen3-embedding-8b?dimensions=4096

But I will wait on maintainer feedback before spending more time on this.

https://www.npmjs.com/package/@erbanku/lat.md

Context: https://github.com/1st1/lat.md/pull/73