Show app count with relative last-refreshed time in subtitle. Auto-refresh
catalog on first visit when empty. Show "Installed" badge on catalog entries
that match already-installed AppImages instead of the Install button.
The AppImageHub feed.json contains null values where arrays/strings are
expected (296 items with null links, 6 items with null inside categories
arrays). Changed AppImageHubItem deserialization to use Option types and
flatten nulls. Also fixed refresh handler to dynamically look up the
catalog source from DB instead of hardcoding id.
