Add keyboard shortcuts for whiteboard tools
The whiteboard toolbar only switched tools by click. Bare P, E and T now switch the pen, eraser and text tools, cycle pen colours or eraser sizes on a repeat press, and restore the previous draw tool when leaving text. Always-visible key labels sit beside those controls so the shortcuts are discoverable, and the shortcuts stay quiet while typing, while a key is held, or when focus is outside the board.
Closes #618
Summary by cubic
Adds keyboard shortcuts for the whiteboard toolbar so bare P, E, and T switch between the pen, eraser, and text tools without clicking. A repeat press cycles pen colors or eraser sizes, and pressing T again while in text mode restores the previous draw tool. Always-visible key labels make the shortcuts discoverable, and shortcuts are ignored while typing, on key repeats, or when focus is outside the board. Closes #618.