The flow editor can now change a flow's shape on the canvas, not only
through the inspector's fields:
- drag a box onto another to move it and its branch there; a drop the
save would refuse shows the target dashed red and gives the reason;
- a palette of node kinds adds a node below the picked box on click, or
below any box it is dragged onto; it offers what the controller lists;
- arrow keys walk the tree, M picks a box up and Enter drops it under
the focused one, Alt+Up/Down reorder a node among its siblings, Delete
removes a branch; focus follows the edit;
- each box with siblings says when it runs, and the inspector can move
it earlier or later, since the router runs children in edge order;
- request_reviewer is a second leaf kind beside todo, with a reviewer
field;
- leaving with unsaved edits asks first, also for in-app links and the
back button (a router hook runs navigation past a page's guard).
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