feat: add UnoRouter as a remote model provider

[0-don]

Describe Your Changes

Adds UnoRouter as a predefined remote model provider, following the shape of the existing OpenRouter entry and the NEAR AI Cloud provider PR (#8169).

UnoRouter (https://unorouter.com) is an open-source OpenAI-compatible gateway serving 200+ models (Claude, GPT, Gemini, DeepSeek, Kimi, GLM, Qwen and more) behind a single API key; models with a :free suffix cost nothing, which gives Jan users working cloud models without a card.

Unlike NEAR AI, UnoRouter uses the standard /v1/models list endpoint, so no custom catalog path or normalizer is needed; the generic OpenAI-compatible paths handle everything.

Changes:

• web-app/src/constants/providers.ts: predefined provider entry (base URL https://api.unorouter.com/v1, api-key setting, two free seed models like the OpenRouter entry)
• web-app/src/constants/models.ts: capability entry (models: true for live catalog, streaming/JSON/images/tools)
• web-app/src/lib/model-factory.ts: unorouter routes to createOpenAICompatibleModel
• web-app/src/lib/utils.ts: provider title + logo
• web-app/public/images/model-provider/unorouter.svg: logo asset
• Docs: docs/.../remote-models/unorouter.mdx + _meta.json nav entry
• Tests: new unorouter-provider.test.ts, plus cases in model-factory.test.ts and utils.test.ts

Self Checklist

• Added relevant comments, esp in complex areas
• Removed debug logs
• Tests pass locally: touched test files 92/92 (unorouter-provider.test.ts, utils.test.ts, model-factory.test.ts)