docs: add ZeroLimitAI custom endpoint provider

[ClawLabsAI]

Summary

• Add ZeroLimitAI to the AI Endpoints documentation and sidebar, following the existing provider-page conventions (same structure as ShuttleAI / Lemonade).
• ZeroLimitAI is an OpenAI-compatible endpoint (https://www.zerolimitai.com/api/v1) that routes each request to the best-ranked free model that is answering, with failover. The page documents the API key, the librechat.yaml block with model: auto and fetch: true, and its limits (streaming and tool calls supported; no embeddings or image input).
• Add a monochrome provider icon (currentColor) and register it in components/icons/providers/index.ts and lib/icons.tsx.

Verification

• Passed pnpm run typecheck.
• Ran the lint-staged steps (prettier --write, eslint --fix) on the changed TS/TSX files: no changes, no warnings.

Disclosure

I maintain ZeroLimitAI. Happy to adjust anything to match the docs' conventions.

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