Persist the highlighted theme as the default for TERM_PROGRAM.
Writes [ui.terminal_themes][TERM_PROGRAM] = name and updates the
(default) badge in the option list without closing the picker, so
the user can confirm the change and keep browsing. [ui].theme is
intentionally not touched because this action saves only the current
terminal default. Config writes are serialized in app.py, so
overlapping global-theme and per-terminal-theme saves cannot clobber
each other's keys.
No-ops with a warning toast if TERM_PROGRAM is unset, or silently
if the option list has no highlighted entry / the highlighted id
isn't a registered theme.
