[Feature Request] docs: PZERO BASE_URL example (do not append /v1)

### 🥰 Feature Description

NextChat already overrides the OpenAI host with `BASE_URL` (README default is `https://api.openai.com`, without `/v1`, because NextChat appends `/v1`). People looking for prepaid, privacy-first inference still have to guess the host, key format, and that adding `/v1` themselves produces `/v1/v1` and 404s.

Search for `pzero` returned none. I am not asking for a first-party PZERO provider.

### 🧐 Proposed Solution

A named PZERO example in the env / custom-endpoint docs.

```bash
OPENAI_API_KEY=pzero_YOUR_KEY
BASE_URL=https://api.pzero.studio
```

Do not set `BASE_URL=https://api.pzero.studio/v1`. NextChat appends `/v1`. The chat path is not the base.

In Settings → custom endpoint, same host (`https://api.pzero.studio`) and a `pzero_` key. Model: `deepseek-v4-flash` (or any live text id from `GET https://api.pzero.studio/v1/models`, public, no key).

Get a key: https://pzero.studio/agents (sign-in free; min top-up 1 USDC on Base; no starter credits).

Smoke against the host NextChat will call after appending `/v1`:

```bash
curl -sS -X POST "https://api.pzero.studio/v1/chat/completions" \
-H "Authorization: Bearer $PZERO_API_KEY" \
-H "Content-Type: application/json" \
-d '{"model":"deepseek-v4-flash","messages":[{"role":"user","content":"Hello from PZERO"}],"stream":false}'
```

If a build does not append `/v1` and 404s on `https://api.pzero.studio/chat/completions`, then that build wants the `/v1` root. The README default (`https://api.openai.com`) is the no-`/v1` shape.

I am not asking for a bundled provider. Do not start implementation on my behalf until the board says Ready; the env vars are for users today.

### 📝 Additional Information

Every AI Credit is at least 20% off $1 list. Usage is paid. Conversion door: https://pzero.studio/agents

Locate the env and custom-endpoint documentation and the README's BASE_URL example. Add a named PZERO example showing the host without /v1, the pzero_ key format, and the model lookup or smoke-test details from the issue. Done means users can configure the endpoint without causing a duplicated /v1 path.

Written by the indexing model from the issue text.