MAC prefix shortlist + IEEE OUI vendor search
Replace the env-var-only RAX_MAC_PREFIXES list with a proper Settings > MAC Prefixes page: operators add/remove their own shortlisted {prefix, label} entries at runtime (persisted in Rax's store, seeded with RAX_MAC_PREFIX_DEFAULT on first run), the same shortlist used by the device page's manual MAC-entry dropdown.
Also adds a vendor search against IEEE's public OUI registry (new internal/ouidb package, same source Wireshark's OUI lookup tool uses) — "Refresh OUI database" fetches and caches it locally, and the search box finds real vendors by name or prefix with a one-click "Add to shortlist".
Co-Authored-By: Claude Sonnet 5 noreply@anthropic.com Claude-Session: https://claude.ai/code/session_01U9buZPRQy7YXLodkSPFFik