test: add initial test files and component stubs

src/containers/TimeConverter.tsx

@@ -0,0 +1,10 @@
+import { useState } from 'react';
+import { BerlinClock } from './BerlinClock';
+import { DigitalClock } from './DigitalClock';
+import { useCurrentTime } from '../hooks/useCurrentTime';
+
+export function TimeConverter() {
+  const [useSystemTime, setUseSystemTime] = useState(false);
+  // When useSystemTime is true, both BerlinClock and DigitalClock are disabled.
+  return null;
+}