fix(app): fall back to document.baseURI when no explicit base path is configured

[awschmeder]

Summary

This PR builds on top of #28326 (runtime base path support for reverse proxy deployments) by adding a zero-config fallback for environments where a reverse proxy injects a <base href> tag into the HTML but the operator cannot or does not pass --base-path to opencode web.

Problem

When opencode is served under a sub-path reverse proxy (e.g., JupyterHub via jupyter-server-proxy), the proxy injects <base href="/user/<u>/opencode/"> into the HTML response. Without --base-path, window.__OPENCODE_BASE_PATH__ is unset and getCurrentUrl() returns location.origin (the domain root). All API calls then go to the origin root instead of the proxy mount point, producing 404 errors.

Fix

In packages/app/src/entry.tsx, after resolving window.__OPENCODE_BASE_PATH__ and VITE_OPENCODE_SERVER_BASE_URL, if basePath is still empty, check document.baseURI. If its pathname diverges from the root, use it as the base path:

if (!basePath) {
  const baseUriPath = new URL(document.baseURI).pathname.replace(/\/+$/, "")
  if (baseUriPath && baseUriPath !== "/") basePath = baseUriPath
}

Priority order:

1. window.__OPENCODE_BASE_PATH__ (set by --base-path, most explicit)
2. VITE_OPENCODE_SERVER_BASE_URL (build-time env var)
3. document.baseURI pathname (proxy-injected <base href>, zero-config fallback)
4. Empty string (origin root, existing behavior)

Behavior

• Direct access (no proxy): document.baseURI pathname is / -- fallback does nothing. No behavior change.
• --base-path /opencode: window.__OPENCODE_BASE_PATH__ is set -- fallback never runs. No behavior change.
• JupyterHub / jupyter-server-proxy: proxy injects <base href="/user/alice/opencode/"> -- fallback detects /user/alice/opencode and uses it as the API base. Works without --base-path.

Relation to #28326

This is an additive change on top of #28326. It does not conflict with the --base-path mechanism -- explicit configuration always wins. The fallback only activates when no base path is configured at all.

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