fix(ui): readable date-time trigger, showcase code panel, release ui 0.8.88

- DatePicker trigger shows the value formatted in the component locale
  ("Oct 2, 2026 · 9:30 AM", "Jul 15, 2026"); the hidden input still submits
  the ISO value.
- Showcase playground code panel: the reactive runtime read "{...}" in the
  rewritten code as interpolations and blanked every object in array props.
  Braces are now written as their own spans, like the server-rendered code.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

packages/ui/test/ui.test.ts

@@ -1687,7 +1687,7 @@ test("date picker renders a custom calendar trigger with accessible field state
   )[14] as HTMLButtonElement;
   fifteenth.dispatchEvent(new (dom.window as any).Event("click", { bubbles: true }));
   expect(input.value).toBe("2026-07-15");
-  expect(dom.querySelector(".wrn-next__date-trigger span")?.textContent).toBe("2026-07-15");
+  expect(dom.querySelector(".wrn-next__date-trigger span")?.textContent).toBe("Jul 15, 2026");
 });
 
 test("time picker uses a custom reactive panel instead of the browser time control", async () => {