Set board mode (draw / todo)

Mirrors the MCP set_board_mode tool. A board is either draw (the default infinite-canvas whiteboard) or todo (a kanban task board with columns and cards). The mode is switchable WHENEVER the board is empty of real content — drawings (text/strokes/images) and tasks. Empty or seeded columns DON'T count (switching to draw clears them), so a board that once held content but is now cleared becomes switchable again, and you can flip draw ↔ todo freely until the first stroke/text/image or task lands. Calling this on a board that still holds real content returns 400 { code: "board_not_empty" }. Switching an empty board to todo seeds the starter columns; pass an optional template (kanban — the default — / sprint / bugs) to choose the column set, returned in the columns array.