feat: Client endpoint for reporting backend API to other instances

[Pecacheu]

Fixes #1293

Required for:

• feat: Multi-account switching & self-hosted/on-premise instances #999

Adds a /.stoat-config endpoint to the client Vite server. Currently this has the API and Gifbox endpoint (other endpoints can already be fetched via backend API, as is done in multiuser PR.)

Response example:

{
    "api": "https://api.stoat.chat",
    "gifbox": "https://api.gifbox.me"
}

How was this PR tested?

Ran client & tested endpoint.

Checklist:

• I have carefully read the contributing guidelines
• I have performed a self-review of my own code
• I have made corresponding changes to the documentation if applicable
• I have no unrelated changes in the PR
• I have confirmed that any new dependencies are strictly necessary
• I have written tests for new code (if applicable)
• I have followed naming conventions/patterns in the surrounding code

Please declare, if any, LLM usage involved in creating this PR

LLMs and their "world models" can't keep up with me, I'm fast as f**k boiiiii
Athletic dodging

• main
  • feat: Client endpoint for reporting backend API to other instances #1308 👈