fix: release DragManager when timeline element is deleted (#515)

[FelipeDefensor]

Closes #515.

Summary

• Clicking a timeline element arms a DragManager that subscribes to TIMELINE_VIEW_LEFT_BUTTON_DRAG / _RELEASE. The subscription was only torn down by DragManager.on_mouse_release, so deleting the element before mouse release left a stale listener whose after_each callback (e.g. MarkerUI.after_each_drag) referred to a now-phantom component. The next drag anywhere in the timeline re-entered that callback on the deleted element and crashed with KeyError on id_to_component.
• Fix is in the base TimelineUIElement.delete: if a drag_manager is attached, unsubscribe it before tearing down the rest of the element. The same orphan path exists for hierarchy, beat and harmony elements (all use the same base delete and the same drag_manager field), so they get the fix for free.
• Adds DragManager.cleanup for the unsubscribe-only case, distinct from on_mouse_release. The latter still fires the user's on_release callback (the on_drag_end state-record post); the deletion path must not, since the Delete command is itself the recorded user action.

Why a base-class fix

The reporter's sketch in #515 is marker-only, but the bug is structural: every element type that calls setup_drag() from on_left_click shares the same listener-leak window. Putting the cleanup in TimelineUIElement.delete (guarded by getattr(self, "drag_manager", None)) covers all current and future kinds without each element having to remember.

Test plan

• New regression test TestDrag::test_delete_mid_drag_does_not_leak_listener in tests/ui/timelines/marker/test_marker_timeline_ui.py reproduces the original repro: click a marker (arms drag_manager), delete it via timeline.component.delete, then post a LEFT_BUTTON_DRAG. Without the fix this raises KeyError: '<marker-id>'; with it, the post is silent.
• Full test suite passes locally (1181 passed, 10 skipped, 2 xfailed, 2 xpassed).

Out of scope

Several on_double_left_click handlers (marker, beat, harmony.harmony, harmony.mode, hierarchy) call self.drag_manager.on_release() to "cancel" a single-click-armed drag. That call only fires the user callback — it does not unsubscribe the listener. In normal flow the LEFT_BUTTON_RELEASE between the two clicks already cleans up, so it isn't observable, but the call is misleading. Left as-is to keep this PR focused on #515.

[azfoo]

@FelipeDefensor you might want to double check this?