Undo, and the toolbar in three groups

Undo takes back one's own latest mark, by the button, Ctrl+Z or two
quick taps of the eraser; the toolbar's drawing tools, the page and
view, and the session stand in three groups that do not move when the
tool changes; the buttons are a little smaller.

client/js/pen_input.js

@@ -29,7 +29,7 @@ const NOMINAL_PRESSURE = 0.4;
  *     Erases one's own strokes and text blocks along the movement and says how many.
  * @property {(x: number, y: number, reach: number) => void} [eraseEnded]
  *     The eraser went down at (x, y) and has come up again, having moved
- *     no farther than `reach` from there.
+ *     no farther than `reach` from there; Infinity if the press was cancelled.
  * @property {(x0: number, y0: number, x1: number, y1: number) => void} placeRect
  *     With the text or Hide tool, the pointer went down at (x0, y0) and up
  *     at (x1, y1): a tap, or a rectangle dragged open.
@@ -292,7 +292,8 @@ export function attachPenInput(surface, target) {
       if (event.type === 'pointerup') target.placeRect(textStartX, textStartY, x, y);
     } else if (erasing) {
       eraseFromPress();
-      if (event.type === 'pointerup') target.eraseEnded?.(eraseStartX, eraseStartY, eraseReach);
+      // A cancelled press ends the gesture too, with no tap in it.
+      target.eraseEnded?.(eraseStartX, eraseStartY, event.type === 'pointerup' ? eraseReach : Infinity);
     } else {
       target.endStroke();
     }