System-level access and IAM page

`GET/PUT /v1/system/policy` handle fleet roles. It just occurred to me that it's weird that we don't have a page analogous to Silo Access on the System side.

<img width="1116" height="747" alt="Image" src="https://github.com/user-attachments/assets/a6fd295a-4c22-42eb-b8a8-ce0366c740d5" />