fix: Size the GCS pairs allocation with the right element type

ui_pairs_init() allocates the per-pair array with
`nbPairs * sizeof(nbgl_contentTagValueList_t)` but g_pairs is typed as
`nbgl_contentTagValue_t *`. The list struct is roughly three times the
size of one tag-value entry, so each pair was being over-allocated by
the size difference, wasting app-memory pool space on every
generic-clear-signing review without any functional benefit.

Use `sizeof(nbgl_contentTagValue_t)` instead so the allocation matches
what the surrounding code reads and writes.

(cherry picked from commit 761c452844b675041384613a3480406dfd5df460)