Tooltips in the Notebooks chooser

From the development repository at e819bc4.

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

client/js/notebook.js

@@ -29,6 +29,13 @@ const LONGEST_WAIT_MS = 2000;
 /** After a write fails, try again this much later. */
 const RETRY_MS = 3000;
 
+/**
+ * A page file in a notebook.
+ * @typedef {object} KeptPage
+ * @property {string} name
+ * @property {number} changed  When it was last written, in milliseconds since the epoch.
+ */
+
 /**
  * What keeps the notebooks: the local program's files, or the browser.
  * @typedef {object} Backend
@@ -37,7 +44,7 @@ const RETRY_MS = 3000;
  * @property {() => Promise<{directory: string, notebooks: string[], trash: boolean}>} listNotebooks
  *     The directory is '' when the notebooks are in the browser; trash
  *     says whether a deleted notebook or page goes to one, or is gone for good.
- * @property {(notebook: string) => Promise<string[]>} listPages  Every page file, Current.svg too.
+ * @property {(notebook: string) => Promise<KeptPage[]>} listPages  Every page file, Current.svg too.
  * @property {(notebook: string, pageName: string) => Promise<string | null>} readPage
  * @property {(notebook: string, pageName: string, text: string) => Promise<void>} writePage
  * @property {(notebook: string) => Promise<{kept: string | null, new: boolean}>} keepPage
@@ -101,7 +108,8 @@ const inFiles = /** @type {Backend} */ ({
   async listPages(notebook) {
     const response = await request('GET', [notebook]);
     if (!response.ok) throw new Error(`The notebook “${notebook}” cannot be read.`);
-    return (await response.json()).pages;
+    const { pages, changed } = await response.json();
+    return pages.map((/** @type {string} */ name) => ({ name, changed: changed?.[name] ?? 0 }));
   },
   async readPage(notebook, pageName) {
     const response = await request('GET', [notebook, pageName]);
@@ -189,8 +197,27 @@ export function discardPage(notebook, pageName) {
  *     notebook, in order; not the page being written.
  */
 export async function listKeptPages(notebook) {
+  return (await listKeptPageDetails(notebook)).map((page) => page.name);
+}
+
+/**
+ * @param {string} notebook
+ * @returns {Promise<KeptPage[]>}  The pages kept in the notebook, in
+ *     order, with when each was last changed; not the page being written.
+ */
+export async function listKeptPageDetails(notebook) {
   const pages = await backend.listPages(notebook);
-  return pages.filter((name) => name !== CURRENT_PAGE);
+  return pages.filter((page) => page.name !== CURRENT_PAGE);
+}
+
+/**
+ * @param {string} notebook
+ * @returns {Promise<number>}  When the page being written in the notebook
+ *     was last written, in milliseconds since the epoch, or 0 if there is none.
+ */
+export async function currentPageChanged(notebook) {
+  const pages = await backend.listPages(notebook);
+  return pages.find((page) => page.name === CURRENT_PAGE)?.changed ?? 0;
 }
 
 /**