Add framesToDuration helper

frontend/src/helpers/framesToDuration.test.ts

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+import framesToDuration from './framesToDuration';
+
+describe('framesToDuration', () => {
+  it('returns the expected result for 0 frames at 44100hz', () => {
+    expect(framesToDuration(0, 44100)).toEqual({ seconds: 0, nanos: 0 });
+  });
+
+  it('returns the expected result for 44100 frames at 44100hz', () => {
+    expect(framesToDuration(44100, 44100)).toEqual({ seconds: 1, nanos: 0 });
+  });
+
+  it('returns the expected result for 88200 frames at 44100hz', () => {
+    expect(framesToDuration(88200, 44100)).toEqual({ seconds: 2, nanos: 0 });
+  });
+
+  it('returns the expected result for 88201 frames at 44100hz', () => {
+    expect(framesToDuration(88201, 44100)).toEqual({
+      seconds: 2,
+      nanos: 22675,
+    });
+  });
+
+  it('returns the expected result for 110250 frames at 44100hz', () => {
+    expect(framesToDuration(110250, 44100)).toEqual({
+      seconds: 2,
+      nanos: 500_000_000,
+    });
+  });
+});

frontend/src/helpers/framesToDuration.ts

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+import { Duration } from '../generated/google/protobuf/duration';
+
+function framesToDuration(frames: number, sampleRate: number): Duration {
+  const secs = Math.floor(frames / sampleRate);
+  const nanos = Math.floor(
+    ((frames % sampleRate) / sampleRate) * 1_000_000_000
+  );
+  return { seconds: secs, nanos: nanos };
+}
+
+export default framesToDuration;