feat: add base path support for reverse proxy deployments

[carsongee]

Closes #25776

What

Adds a base path option so opencode web works correctly when served behind a reverse proxy that adds a path prefix.

Three ways to configure it:

• --base-path /my/prefix CLI flag
• OPENCODE_SERVER_BASE_PATH=/my/prefix env var
• server.basePath in opencode.json

Why

Asset URLs in the built HTML use root-relative paths (/assets/index-xxx.js). Behind a path-prefixed proxy the browser resolves them against the host root, not the proxy prefix, causing 404s for all JS/CSS.

How it works

When basePath is set, the server injects <base href="/the/prefix/"> into every HTML response (both embedded and proxied). Browsers then resolve all asset and navigation URLs relative to that base — no rebuild required. The SolidJS Router reads the <base> element at boot and uses its pathname as its base prop so client-side routing stays within the prefix.

Verification

Start with a prefix:

opencode web --port 4096 --base-path /notebook-sessions/abc/ports/4096

Confirm curl -s http://localhost:4096/ | grep '<base' returns <base href="/notebook-sessions/abc/ports/4096/">, and that all assets load when accessing via the proxy URL.

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[github-actions]

The following comment was made by an LLM, it may be inaccurate:

Potential Duplicates Found

There are existing PRs addressing similar functionality:

1. PR feat: support serving opencode from a subpath #25513 - "feat: support serving opencode from a subpath"

   • feat: support serving opencode from a subpath #25513
   • Directly related: addresses serving from a subpath, which is the same core issue as base path support

2. PR feat: make opencode web embeddable in iframes at a subpath #23912 - "feat: make opencode web embeddable in iframes at a subpath"

   • feat: make opencode web embeddable in iframes at a subpath #23912
   • Related: handles subpath deployment scenarios

3. PR feat: base path support #7625 - "feat: base path support"

   • feat: base path support #7625
   • Potential historical duplicate with the same feature name

Recommendation: Check PR #25513 and #23912 to see if they address the same reverse proxy path prefix issue, particularly around asset URL resolution and the <base href> injection approach.

[carsongee]

Closing in favor of the existing PR addressing #7624.