Applied gtk_box_set_homogeneous() in GTK3 branches where the GTK2 code requested homogeneous layout (e.g., dialog hboxes).
Updated remaining helpers and setup/preferences UI box constructors (including the shared mg_box_new helper) to follow the same GTK3/GTK2 branching approach consistently across src/fe-gtk/
- Replaced the dark mode checkbox with an Auto/Dark/Light color mode selector and ensured palette edits use the resolved mode.
- Applied the resolved color mode consistently across palette saving and GTK styling in the user list and channel tree/theme application paths.
- Removes need to check for malloc failure
- Removes need for NULL checks on free
- Adds checks for integer overflows
- Removes some extra memset calls
- Removes chance of mixing libc and glib malloc/free
