test(app): add mock llm e2e fixture

[kitlangton]

Summary

• add a Playwright llm fixture backed by the shared TestLLMServer so app e2e tests can use the same mock provider abstraction as the opencode tests
• extend withProject(...) so e2e tests can seed a temp workspace with mock-provider config and model selection before navigation
• migrate the prompt reply smoke test to the mock LLM server and verify it runs end-to-end without hitting a real provider