Fix(eip712): pin hash display strings in signing context

eip712_format_hash() was storing NBGL display pointers directly into
strings.tmp.tmp, a scratch buffer shared with every other signing flow.
Any APDU processed while the EIP-712 review is displayed (the APDU loop
stays alive on SWO_NO_RESPONSE) could silently overwrite the domain/
message hash strings without NBGL noticing.

(cherry picked from commit d817f91b0ba1367a5235f830ec90d98942c5104f)