Mobile contracts

Mobile reference pages cover durable contracts shared by Ferrex clients.

Page	Status
Mobile device-auth FlatBuffers contract	Current contract note. Server device-auth endpoints still use JSON handlers, so FlatBuffers server negotiation remains follow-up work.

Keep generated Kotlin/Rust bindings and schema changes tied to the codegen command recorded in the relevant implementation issue. Do not treat a contract page as proof that generated code was refreshed unless its validation section says so.