update() guarded the per-folder timer with `i in self.timer_folders`,
which asks whether the integer i is one of the Timer objects in the list.
Timer does not implement __eq__, so that test is always False: the skip
branch was unreachable and the reset branch never ran.
The consequence is that folder_N_refresh has no effect. Every folder is
re-walked on every refresh cycle (2s by default) rather than on its own
schedule, and folder_size() is a full recursive walk - which is exactly
why the setting exists.
Use the index bound the code meant, `i < len(self.timer_folders)`. The
guard still earns its place: the folder list lives on the class, so it
can outgrow the per-instance timer list.
