chore: remove dead resource type re-export

[shuxueshuxue]

Summary

• remove the dead SessionMetrics type re-export from frontend/app/src/pages/resources/SandboxDetailSheet.tsx
• drop the now-unused SessionMetrics import from the same file
• keep the live type source in frontend/app/src/pages/resources/types.ts

Residual Risk

• if something outside this repo imports SessionMetrics from SandboxDetailSheet.tsx directly, that repo-external usage would now break

Test Plan

• ALL_PROXY= HTTPS_PROXY= HTTP_PROXY= all_proxy= https_proxy= http_proxy= uv run pytest tests/Integration/test_invite_codes_router.py tests/Integration/test_thread_files_channel_shell.py tests/Integration/test_settings_local_path_shell.py -q
• cd frontend/app && npm run build