Share one selection menu; drop dead variant detection and TegraExplorer filter

The variant menu in main.c and the RAM / Hekate-AMS menus in install.c
were three copies of the same input loop; install_menu_select() replaces
them with identical behaviour (D-pad/Vol to move, A/Power to confirm,
Vol+ and Vol- to abort, Capture for screenshots).

source/install.h

@@ -37,6 +37,8 @@ void install_preserve_update_cleanup_end(void);
 
 // Shared helpers (install.c) - used by install_update.c and install_clean.c
 void install_set_color(u32 color);
+// Shared selection menu: returns index or -1 on Vol+/Vol- abort
+int install_menu_select(const char *const *labels, int n);
 void install_check_and_clear_screen_if_needed(void);
 bool install_path_exists(const char *path);
 int install_move_staging_root_files(const char *staging, const char *dst_root);