WebUI unusable behind a path-prefix reverse proxy (renders its own 404; API calls escape the prefix)

[SelfRef]

Use case

llama-swap (https://github.com/mostlygeek/llama-swap) can manage audiocpp_server
as an on-demand TTS backend: it starts/stops one instance per configured model and
proxies it under a path prefix, e.g. https://host/upstream/<model>/. The OpenAI
endpoints work great this way - but the embedded WebUI does not.

Problem

The WebUI is a SvelteKit app built for the domain root:

1. Pathname routing - the document is served fine at
   /upstream/<model>/ (HTTP 200), but the client-side router resolves the
   current location.pathname, finds no matching route, and renders the
   app's own 404 page. So any path-prefix deployment shows "404" even though
   the server is healthy.
2. Root-absolute API calls - src/lib/api.ts fetches /health,
   /v1/models, /v1/audio/speech, … with absolute paths, so behind a prefix
   they escape to the proxy root instead of reaching the instance.

What fixed it for me (tested)

• svelte.config.js: kit.router = { type: 'hash' } - the pathname stops
  mattering, and paths.relative: true (already set) keeps assets working.
  Requires dropping the prerender/ssr page options from
  src/routes/+page.ts (hash router rejects them at build time).
• Rebasing the root-absolute fetches onto the page's directory. I used a
  small window.fetch wrapper in app.html (no-op when served at a real
  root); making the api.ts paths relative would be the cleaner in-tree fix.

With those two changes the full UI (model load, TTS, cloning) works behind
/upstream/<model>/ with zero proxy configuration. Happy to send a PR if
you're open to either approach.

[0xShug0]

@SelfRef Thanks for reporting this issue and PRs are very weclome!