swarm-ui try-out: stable live region, reasons for disabled buttons

The running line is a live region whose text changed every second, so a
screen reader announced the counter again each time. The sentence is now
stable and the elapsed seconds sit outside it.

The run and open-a-PR buttons point at the hint that says why they are
off, and add-a-classification says why it is off at the cap.

frontend/packages/swarm-ui/test/tryout-types.test.mjs

@@ -5,6 +5,8 @@
 import assert from "node:assert/strict";
 import { test } from "node:test";
 import {
+  MAX_CLASSIFICATIONS,
+  addProblem,
   draftFromRule,
   draftProblem,
   duplicateName,
@@ -17,6 +19,12 @@ import {
   toBody,
 } from "../src/pages/review-tryout/TryOutTypes.ts";
 
+test("a row is offered until the endpoint's cap is reached", () => {
+  assert.equal(addProblem(1), null);
+  assert.equal(addProblem(MAX_CLASSIFICATIONS - 1), null);
+  assert.match(addProblem(MAX_CLASSIFICATIONS), /at most 10 classifications/);
+});
+
 // A row ready to send, with `over` replacing any of its fields.
 function row(over = {}) {
   return {