Support serving the GUI behind a reverse-proxy sub-path

[Slav4ik]

Fixes #3

Summary

Makes the WireMock GUI work when served behind a path-based reverse proxy (e.g. https://host/wiremock/__admin/gui) without requiring a custom rebuild or fork.

Changes

1. Relative asset paths + runtime <base href> (vite.config.ts, index.html)

• Switched Vite base from the hardcoded absolute /__admin/gui/ to './', so the build emits relative asset references (./assets/...).
• Added a small inline script in index.html that runs before the module script and injects a <base href> derived from window.location.pathname up to and including /__admin/gui/.

This ensures relative asset URLs resolve against the actual page location, not the host root.

2. API base URL prefix preservation (App.tsx)

• Updated getDefaultBaseUrl() to return origin + <prefix> where <prefix> is the pathname segment before /__admin.
• Example: page at /wiremock/__admin/gui → API base https://host/wiremock (requests go to https://host/wiremock/__admin/...).

Existing localStorage override and http://localhost:8080 dev fallback are unchanged.

Backward compatibility

Root-mounted deployments (https://host/__admin/gui) are unaffected:

Scenario	<base href>	Asset ./assets/x.js resolves to	API base
Root mount /__admin/gui	/__admin/gui/	/__admin/gui/assets/x.js	https://host
Prefixed mount /wiremock/__admin/gui	/wiremock/__admin/gui/	/wiremock/__admin/gui/assets/x.js	https://host/wiremock

In the prefixed case, the browser sends requests with the /wiremock prefix; the reverse proxy strips it before forwarding to WireMock, which still sees /__admin/gui/assets/... and /__admin/....

Build verification

npm ci && npm run build   # webapp/

Emitted dist/index.html (excerpt):

<script>/* runtime <base href> injection */</script>
<script type="module" crossorigin src="./assets/index-Fp2cjh1i.js"></script>
<link rel="stylesheet" crossorigin href="./assets/index-CjuQgt2G.css">

Asset paths are relative (./assets/...), not absolute (/__admin/gui/assets/...). Combined with the runtime <base href>, they resolve correctly for both root and prefixed mounts as shown in the table above.

Test plan

• Load GUI at http://localhost:8080/__admin/gui — assets load, API calls succeed
• Load GUI at http://localhost:8080/wiremock/__admin/gui via a reverse proxy that strips /wiremock — assets load, API calls succeed
• Vite dev server (npm run dev) still works for local development

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