Improve whiteboard density and callouts

Fix current-source lessons and compact board layout

Enrich whiteboard diagrams and responsive board

no more slivers: min shape dims + vertical label fit

polygon labels fit the shape interior (chord-width shrink)

board polish: highlights hug measured text, arrows chase displaced labels, hypotenuse label on longest side

fixes: shrink-to-fit shape labels, dark-mode-proof text, responsive stack, coach echo guard

v2: teacher's-studio design, MiniCPM5 planner+coach, expanding board, field notes

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