chore: remove dead thread list fallback

[shuxueshuxue]

Summary

• remove the dead toThreads() compatibility parser from frontend/app/src/api/client.ts
• have listThreads() read the current /api/threads contract directly as { "threads": ThreadSummary[] }
• keep the change net-negative and frontend-only

Residual Risk

• if some stale backend outside this repo still returns a bare array instead of { "threads": [...] }, that unsupported version skew would now fail on the frontend

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