feat: Implement UI for Local Engine Management

[hantran-co]

Problem Statement

• A sub-task of: roadmap: Jan Local Engine Management can pick Version and Variant #4128

https://cortex.so/api-reference#tag/engines/get/v1/engines/{name}

Jan needs a user interface to manage Cortex.cpp engines and their variants. This includes viewing installed engines, managing engine installations, and controlling engine settings through a unified interface.

Acceptance Criteria

1. Users can view all installed engines and their variants
2. Users can set and change default variants
3. Users can update engines to latest versions
4. Only one engine can be active at a time
5. UI is responsive and provides feedback for all operations
6. All error states are properly handled and communicated

Design

Figma link: https://www.figma.com/design/DYfpMhf8qiSReKvYooBgDV/Jan-App-(3rd-version)?node-id=9273-110339&m=dev