getMaintenanceUf2Manifest

Fetches the pinned maintenance-UF2 manifest (factory-erase and OTAFIX bootloader self-update images) from the Meshtastic API. Decodes directly to the same model the bundled asset seed uses — the server serves this file verbatim, so there is nothing to transform. Each image's own sha256 is still checked against the downloaded bytes before any write; that is unaffected by how this manifest itself is fetched.