Fix drag, drop, and side panel bugs in the production schedule

[NicoPennec]

Problem

• Dragging an episode bar over another row of the all-episodes planning throws on every mouse move and freezes the drag.
• Dragging a multi-assignee task across several person rows unassigns an untouched co-assignee and keeps the intermediate row assigned.
• Drops from the assignment panel land on the wrong date: week cells are counted as days, and the cursor offset is hardcoded.
• Saving a schedule item without an end date shifts its start date by one day at every save.
• Closing the task side panel takes two clicks, and the task keeps a selection whose resize handles swallow the next click.
• A watcher observes a height prop that was removed from the widget.
• The unassign handler can splice the wrong bar on a lookup miss, and the drill-down error fallback resets people to the wrong type.
• Versioned tasks created from the assignment flow lose their ID, so the next edit posts a duplicate.
• The task panel hides out-of-department assignees, and saving silently unassigns them.
• Picking a task type hidden by the entity filter expands a row that the schedule does not show.

Solution

• Disable the row reassignment on the all-episodes planning and guard checkUserIsAllowed against non-person rows.
• Track the person row each dragged bar last hopped to and unassign that one; skip items already on the target row.
• Resolve the hovered column like the position bar does, from the wrapper rect and the day/week columns.
• Clone the start date before computing the fallback end date, on create and update.
• The close and cancel buttons close in one click and clear the schedule selection (resetSelection is now exposed).
• Drop the dead watcher.
• Guard the splice with an index check and reset people to an object.
• Report the created ID on the versioned task after the POST.
• Keep persons already assigned to the edited task listed, whatever their department.
• Clear the entity filter when the selected task type is not visible.