App tests for Shortest

[slavingia]

Put tests next to the page.ts (same folder) so we don't need a specific app/tests/ folder.

Requires updating testDir to testPattern:

import type { ShortestConfig } from "@antiwork/shortest";

export default {
  headless: false,
  baseUrl: "http://localhost:3000",
  testPattern: "*.test.ts",
  anthropicKey: process.env.ANTHROPIC_API_KEY,
} satisfies ShortestConfig;

Include an example test that makes sure that buttons are rounded.