Use local files for Dashboard 2.0 App Icon

### Description

When the server IP or domain name is unique and fixed, 'httpStatic' can be used; But if the server has multiple IPs or domains, it needs to be able to directly call local files or use relative paths based on the local machine (rather than absolute paths starting with HTTP).

### Have you provided an initial effort estimate for this issue?

I am no FlowFuse team member

Start by tracing how the Dashboard 2.0 App Icon is configured and how httpStatic handles paths. Confirm the behavior with a server that has multiple IPs or domains, and consider the work done when the icon can resolve from local files or relative paths rather than only absolute HTTP paths.

Written by the indexing model from the issue text.