Add a compact "slim" card variant and a 4-column grid section that
renders it, used for the Popular albums row on Discover Home.
- SlimCard: 60px row with optional rank, 44px thumbnail, title/subtitle
- GridSection: 4-column grid; cards absolutely positioned since Slint
GridLayout cannot host a `for`
- state.slint: SlimItem struct and HomeState.popular model
- home.rs: load_home now returns HomeData { sections, popular };
map_slim maps the discover most-streamed container
- artwork pipeline: ArtworkTarget enum routes decoded covers to either
a section card or a popular card
cc
