chore: drop unused resource session helpers

[shuxueshuxue]

Summary

• remove unused useSessionCounts() and groupByLease() exports from frontend/app/src/pages/resources/session-list-utils.ts
• prune the now-dead useMemo import and STATUS_ORDER constant
• keep the live LeaseGroup type export for SandboxCard and SandboxDetailSheet

Residual Risk

• if something outside this repo imports those deleted page-local helpers 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