Fix unit visibility change not propagating to other views (#264)

`get_visible_unit_ids()` returned a direct reference to the internal
`_visible_unit_ids` list. Callers that snapshot the list before and after
`set_unit_visibility()` to detect changes were comparing the same mutated
object, so the comparison always found no difference and never triggered
`notify_unit_visibility_changed()`.

Return a copy instead so before/after comparisons work correctly.

Co-authored-by: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: xiao-1011

spikeinterface_gui/controller.py

@@ -600,7 +600,7 @@ def set_visible_unit_ids(self, visible_unit_ids):
 
     def get_visible_unit_ids(self):
         """Get list of visible unit_ids"""
-        return self._visible_unit_ids
+        return list(self._visible_unit_ids)
 
     def get_visible_unit_indices(self):
         """Get list of indices of visible units"""