feat: Implement UI for Remote Engine Management

[imtuyethan]

Problem Statement

• A sub-task of roadmap: Jan has revamped Remote Engines (e.g. OpenAI, Anthropic etc) #3786

Jan needs a user interface to manage Remote API integrations (OpenAI, Anthropic, etc.) that will work with Cortex's new routing system for /chat/completion requests. The UI should support model discovery, API key management, and configuration settings for each provider.

Acceptance Criteria

1. Users can add and configure multiple remote API providers - pending design flow from @imtuyethan
2. API keys are securely stored and managed
3. Available models are correctly displayed and can be enabled/disabled
4. Provider-specific settings are properly saved and applied
5. Error states are properly handled and communicated
6. Changes in provider/model settings are immediately reflected
7. UI remains responsive during API operations

Design

Figma link: https://www.figma.com/design/DYfpMhf8qiSReKvYooBgDV/Jan-App-(3rd-version)?node-id=9276-43132&m=dev

Adding a new remote engine: https://www.figma.com/design/DYfpMhf8qiSReKvYooBgDV/Jan-App-(3rd-version)?node-id=9510-125477&m=dev

[louis-jan]

Action items:

1. How to setup models for customized providers which do not support /models endpoint?

Populate from apps still

2. How to transfer existing settings?

Populate while creating on launch

[dan-menlo]

• Remote Engines that don't have /models
  • Most of them (outside of OpenRouter and OpenAI)
  • e.g. groq
  • Solution: we import it from Jan first (API key, model list)
• Long-term
  • Design + Architecture Request: Let user input model id (customized ID) and settings @imtuyethan
  • Don't try to maintain everything, allow user to have flexibility