fix: Lock EIP-712 type system at FILT_ACTIVATE to prevent post-hash schema injection

handle_eip712_filtering() under P2_FILT_ACTIVATE computes the schema
hash that downstream filter signatures are issued against, but left
the EIP-712 type system unlocked: the host could continue sending
INS_EIP712_STRUCT_DEF APDUs after the activate, append fields or
even entire structs to the schema, and then sign a typed message
whose actual schema differs from the one the filter metadata was
issued for.

Compute the schema hash first, and only on success switch the UI
into filtering-full mode and set struct_state to DEFINED so
subsequent handle_eip712_struct_def() calls are refused by the
existing gate at commands_712.c:94. The whole branch is inside the
`!N_storage.verbose_eip712` check (verbose mode never computes the
hash). All three state changes (hash, filtering mode, type-system
lock) move together, so a transient compute_schema_hash() failure
leaves the handle clean and the host can retry the activate.

(cherry picked from commit bc18ee062adf0a821656701c7a6faf1f3a790ad7)