feat: probe and use local gateway if available

[2color]

Description

This PR checks if there's a local trustless gateway available (from IPFS Desktop or Kubo running on the user's machine) and adds it to the configuration.

Fixes #299

Adding it to the config on load is convoluted due to the way we persist and pass config from across origins into service workers.

Notes & open questions

• In all my tests, I haven't been able to get the service worker to actually make requests to the local gateway. I couldn't find any such restriction in the service worker docs.

Change checklist

• I have performed a self-review of my own code
• I have made corresponding changes to the documentation if necessary (this includes comments as well)
• I have added tests that prove my fix is effective or that my feature works

[lidel]

Continued in #303