Match Cloud folder reordering to the sidebar and fix stuck drags (#12589)

* fix: expose owning WebView lookup to shared undo routing

* test: avoid NSWindow keyWindow collision in Cloud drag fixture

* fix: stabilize Cloud row view compilation

* test: reproduce Cloud folder drop and leading identity layout failures

* fix: retarget Cloud folder drops and compact leading row identity

* test: reproduce Cloud drag feedback and stuck destination teardown

* fix: share sidebar drag feedback and finish Cloud destination sessions

* test: require Cloud reorder to appear before drag completion

* fix: apply accepted Cloud reorder before source drag completion

* test: cover visible Cloud reorder and reused leading-pin cells

* test: expose isolated Cloud sidebar fixture for remote interaction