docs: add PowerTokens custom endpoint guide

[PowerTokens]

Adds a PowerTokens page under AI Endpoints, in the same format as the NEAR AI Cloud page (#668).

I'm on the PowerTokens team. PowerTokens is an OpenAI-compatible gateway (https://api.powertokens.ai/v1) for GLM, MiniMax, Qwen and DeepSeek models, with one API key for every model.

Changes:

• content/docs/configuration/librechat_yaml/ai_endpoints/powertokens.mdx: key setup and a endpoints.custom example.
• meta.json: page entry, alphabetical.
• components/icons/providers/powertokens.tsx + lib/icons.tsx: provider icon (monochrome, currentColor).

pnpm typecheck, pnpm lint, and pnpm lint:prettier were not run in this environment. The page was not clicked through inside the LibreChat app. The YAML uses the documented custom-endpoint shape and the same base URL PowerTokens publishes for chat completions: https://docs.powertokens.ai?utm_source=github&utm_medium=librechat&utm_campaign=librechat

Happy to keep the page up to date. Contact: service@powertokens.ai

[vercel]

@uselesssoso is attempting to deploy a commit to the LibreChat's projects Team on Vercel.

A member of the Team first needs to authorize it.