test: add initial test files and component stubs

src/containers/BerlinClock.tsx

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+import { useState } from 'react';
+import { LampLine } from '../components/LampLine';
+import { useCurrentTime } from '../hooks/useCurrentTime';
+import { dateToBerlin } from '../utils';
+
+interface BerlinClockProps {
+  enabled: boolean;
+}
+
+export function BerlinClock({ enabled }: BerlinClockProps) {
+  const { currentTime } = useCurrentTime();
+  const berlin = dateToBerlin(currentTime);
+  // Will render four LampLine components
+  return null;
+}

src/containers/DigitalClock.tsx

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+import { useState } from 'react';
+import { useCurrentTime } from '../hooks/useCurrentTime';
+import { dateToDigital } from '../utils';
+
+interface DigitalClockProps {
+  enabled: boolean;
+}
+
+export function DigitalClock({ enabled }: DigitalClockProps) {
+  const { currentTime } = useCurrentTime();
+  const digital = dateToDigital(currentTime);
+  return null;
+}

src/containers/TimeConverter.tsx

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+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;
+}