chore: remove dead invite code fallback

[shuxueshuxue]

Summary

• remove the dead Array.isArray compatibility branch from fetchInviteCodes()
• prune the now-unused list_all* fake-repo surface from test_invite_codes_router.py
• keep the invite-code helper's real error-contract tests (500 prefix mapping, HTTPException passthrough, revoke -> 404)

Residual Risk

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

Test Plan

• uv run ruff check tests/Integration/test_invite_codes_router.py
• uv run ruff format --check tests/Integration/test_invite_codes_router.py
• 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