fix: Free home-screen plugin tagline before reallocating

get_appname_and_tagline() allocates g_tag_line on every home-screen
transition where the caller is a plugin, but nothing freed the prior
buffer first ? the comment at the allocation point even acknowledged
"will never be deallocated". Each return to the home screen during a
plugin-driven session (post-tx, post-cancel, multi-flow plugins)
therefore leaked roughly the plugin name length plus the tagline
template into the shared app-memory pool, slowly starving the rest
of the session.

Release the previous allocation with APP_MEM_FREE_AND_NULL() before
the next APP_MEM_CALLOC() so the pool only ever holds one live
tagline buffer.

(cherry picked from commit 113b0fa5b1f4828146c0e28bca49d4397c000386)