Everyone has the same buttons; the view is personal

The host's rights and the co-host are gone: anyone in a session may
start a new page, put an image on the page, load a kept page, hide,
expand and shrink, and move or remove images and Hides; the page left
is kept in every notebook.  Fit and Normal scale act on your own screen
only.  A session goes on without whoever started it, and a newcomer to
an empty room starts page 1 with New.

Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>

client/js/page_model.js

@@ -338,8 +338,8 @@ function withoutData(image) {
 }
 
 /**
- * What identifies a page and gives its size.  The host sends it to the
- * others when the page changes.
+ * What identifies a page and gives its size.  Whoever starts or changes
+ * a page sends it to the others.
  * @typedef {object} PageDescriptor
  * @property {string} id
  * @property {number} number  Counts the pages of a session from 1.
@@ -504,8 +504,8 @@ export class Page {
   /**
    * @param {number} [width]
    * @param {number} [height]
-   * @param {string} [id]      Empty for the placeholder a guest shows
-   *                           before the host's page arrives.
+   * @param {string} [id]      Empty for the placeholder someone joining
+   *                           shows before a page arrives.
    * @param {number} [number]  0 for the placeholder.
    */
   constructor(width = BLANK_PAGE_WIDTH, height = BLANK_PAGE_HEIGHT, id = '', number = 0) {
@@ -781,9 +781,10 @@ export class Page {
   }
 
   /**
-   * Layer ids from bottom to top.  The host's layer is on top so that
-   * corrections lie over the writing they correct.
-   * @param {string} hostId
+   * Layer ids from bottom to top.  The layer of whoever started the
+   * session is on top, so that a teacher's corrections lie over the
+   * writing they correct (a drawing order, not a right).
+   * @param {string} hostId  Who started the session.
    * @returns {string[]}
    */
   layerOrder(hostId) {