fix: Strip uint256 zero padding before rendering EIP-712 datetime values

ui_712_format_datetime() decoded the timestamp with
u64_from_BE(data, length), which reads at most 8 bytes from the
start of the buffer. EIP-712 typically declares timestamps as
uint256, so a host-supplied 32-byte value carries its trailing 8
bytes as the meaningful payload and zero-pads the first 24. The
formatter therefore read 8 bytes of padding, displayed 1970-01-01,
and let the user sign a hash computed over the actual timestamp.

Detect that the buffer is wider than uint64_t, verify the leading
bytes are zero, and advance the data pointer past the padding so the
decoded timestamp matches what is being hashed. Non-zero leading
bytes mean the value cannot fit a time_t — refuse the screen in
that case rather than wrap to an arbitrary date.

(cherry picked from commit c09c0e48492646a7087e533c254c8846f92e1148)