Allow setting proxy different from system proxy

### Feature summary

Override HTTP_PROXY/HTTPS_PROXY

### What problem are you trying to solve?

Many corporate networks use proxy authentication which can be bypassed by running software like PX locally.
However, you don't always want to route ALL traffic through PX - thus, like in VS Code, it can be very helpful to override the system proxy setting for this app only.

### Proposed solution

Allow setting proxy settings like in VS Code, i.e.

### Workflow impact

Users behind enterprise proxies

### Installation context

_No response_

### Additional context

_No response_

No files or tests are named. Start by locating the app's handling of HTTP_PROXY/HTTPS_PROXY and system proxy settings, then compare the requested per-app behavior with VS Code's proxy configuration. Done means users can set an app-only proxy without changing system-wide traffic, including authenticated corporate-proxy use.

Written by the indexing model from the issue text.