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/text_editor.js

@@ -30,6 +30,8 @@ export class TextEditor {
    * @param {object} target
    * @param {(block: TextBlock) => void} target.setText   The block changed.
    * @param {(id: string) => void} target.removeText      The block was left empty.
+   * @param {(id: string) => void} [target.beginEdit]     Editing of the block begins: one gesture until endEdit.
+   * @param {(id: string) => void} [target.endEdit]       The editing ended.
    * @param {(clientX: number, clientY: number) => boolean} [target.beginHold]
    *     The pen rested on the field for HOLD_MS: a hold, as on the page
    *     (Andy, 2026-10-09: the box being typed in can be moved without
@@ -185,6 +187,7 @@ export class TextEditor {
    */
   edit(block) {
     this.stop();
+    this.target.beginEdit?.(block.id);
     this.block = block;
     this.renderer.editingText = block.id;
     this.field.value = block.text;
@@ -207,6 +210,7 @@ export class TextEditor {
     this.field.value = '';
     if (block.text.trim() === '') this.target.removeText(block.id);
     else this.target.setText(block);
+    this.target.endEdit?.(block.id);
   }
 
   /**