allow overriding web client deployment path / bundle web client with `opencode` binary.

[paperclover]

Question

https://github.com/sst/opencode/blob/1658a3ff59b9ebacb95221fab69b4d3cecca08fd/packages/opencode/src/server/server.ts#L2629

this is both a stability and privacy concern.

• i've had situations where my docker deployment of an opencode server fails to hit this proxy url sometimes. also, if this hosted deployment gets an update that isn't compatible with the current server, it will break.
• c.req.path includes the project's path, leaking it to the external server. this isn't a big deal for me since i work on projects that are publicly accessible

i found this because i was trying to add a patch to the web interface, with the first question being "how do i run it in dev mode". for contributing, it would be very useful if this url could be overridden to always refer to the local bundle copy.

can that be done? i'd be fine opening a pr to sort that out (additionally making sure that dev mode works; it appears to not work when replacing the URL)