feat(ui): simplify custom model and Omni collection management

[fl0rianr]

GUI3: simplify custom model and Omni collection management

Summary

• replace the bulky custom-model actions with an accessible settings icon next to the Models heading
• provide a unified editor for custom models and Omni collections
• simplify Omni collection setup and move tools and the system prompt under Advanced settings
• keep saved collections editable with focused Settings and Files views
• update accessibility tests for the new navigation and editor structure

Testing

• local ui testing
• all npm test passed

Closes #2563