feat: scroll kanban columns independently

[oobagi]

Summary

• make active kanban columns scroll vertically on their own instead of growing the whole board
• keep the focused card inside a padded column camera and preserve headers when entering from below
• cap tall active kanban boards so surrounding note content remains visible for context

Test plan

• go test ./internal/editor
• HOME="/var/folders/2z/hhv30x310gb6fv8jsq0s7x400000gn/T/tmp.H99lyxSbEg" go test ./...
• go vet ./...