[codex] Add TokenLab provider

[hedging8563]

Summary

• add TokenLab as a first-class provider with the OpenAI-compatible base URL
• route chat completions and native Responses API calls through TokenLab
• expose TokenLab in the self-hosted provider picker with default model gpt-5.4-mini and docs link

Validation

• git diff --check
• npx prettier --check src/globals.ts src/providers/index.ts src/providers/tokenlab/api.ts src/providers/tokenlab/index.ts src/public/index.html
• npm ci --ignore-scripts
• npm run build

Note: npm run build exits 0, while the repository currently emits pre-existing Rollup/TypeScript warnings unrelated to this provider addition.