[FEATURE]: Base path / prefix routing support

[hsteude]

Feature hasn't been suggested before.

• [x] I have verified this feature I'm about to request hasn't been suggested before.

Describe the enhancement you want to request

Use case

We'd like to integrate OpenCode as a feature into larger platforms, where it runs under a URL prefix (e.g., platform.com/opencode/) rather than at the root path.

Request

Add support for a configurable base path so OpenCode can be embedded into existing platforms and served alongside other applications under the same domain.