diigo-cache: public list shows each bookmark's date first

The list is in diigo's updated order; each item now leads with its
updated_at as Date#toISOString() (diigo sends epoch seconds; updatedISO
converts, tested), or "(no date)".

Generated-by: Claude Opus 5.5

diigo-cache/tests/caches.test.ts

@@ -117,6 +117,8 @@ describe("worker routes", () => {
 		const again = await (await call(`/public/${username}?page=2`)).text();
 		expect(again).toContain("<strong>cache: HIT");
 		expect(html).toContain("bookmark 25");
+		// the date comes first, in toISOString() form
+		expect(html).toMatch(/<li><time datetime="\d{4}-\d\d-\d\dT[\d:.]+Z">[^<]+<\/time> <a /);
 		expect(html).not.toContain("bookmark 27"); // private (27 % 3 === 0)
 
 		const json = (await (