fix(eip712): track PREPARING state and restrict ui_712_init bzero scope

eip712_context_init() now sets appState = APP_STATE_PREPARING_EIP712
on success so that handle_first_sign_chunk()'s APP_STATE_IDLE guard blocks
any interleaved INS_SIGN during the struct-definition phase.

ui_712_init() restricts its explicit_bzero to strings.tmp.tmp instead of
the whole strings union; the EIP-712 init path has no business clearing
strings.common, which may belong to another flow.

(cherry picked from commit 7363509c1dbda1871824fc4ff9b2eb6e3495d104)