feat: add UnoRouter provider

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Description

Adds UnoRouter as a model provider. UnoRouter (unorouter.com) is an open-source OpenAI-compatible gateway serving 200+ models (Claude, GPT, Gemini, DeepSeek, Kimi, GLM, Qwen and more) behind one API key; models with a :free suffix cost nothing.

What's included, mirroring the Xiaomi MiMo provider PR (#9404):

• core/llm/llms/UnoRouter.ts: OpenAI subclass, apiBase https://api.unorouter.com/v1/, default model deepseek-v4-flash:free
• Registered in core/llm/llms/index.ts, PROVIDER_HANDLES_TEMPLATING and PROVIDER_SUPPORTS_IMAGES in core/llm/autodetect.ts
• Subclass tests in core/llm/llms/OpenAI-compatible.vitest.ts (218/218 pass locally)
• Docs page docs/customize/model-providers/more/unorouter.mdx + docs.json nav entry
• GUI Add Model entries (providers.ts, models.ts) + logo

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Checklist

• I've read the contributing guide
• The relevant docs, if any, have been updated or created (docs/customize/model-providers/more/unorouter.mdx)
• The relevant tests, if any, have been updated or created (core/llm/llms/OpenAI-compatible.vitest.ts)

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