Fix: Show placeholder icon when item has no thumbnail

[hyldmh]

Fix: Shows with no thumbnail appear as broken images

Closes #43

Problem: When an item has no thumbnail, the code returned an empty <View style={style} /> — a transparent, invisible box. Users see this as a "broken image".

Fix: When uri is undefined (no thumbnail stored), render a broken-image MaterialIcons placeholder instead of an empty View.

// Before
if (!uri) {
  return <View style={style} />;
}

// After
if (!uri) {
  return (
    <View style={style}>
      <MaterialIcons name="broken-image" size={Math.min(style.width, style.height) * 0.5} color="#999999" />
    </View>
  );
}

[Mossop]

Thanks but this makes a change to the wrong package. packages/cli/resources/thumbnail.js is probably where the issue is.