Grid picker cells: revert uneven padding, uniform 3px margins

[Linuxuser67]

Supersedes #478. Uniform 3px margins on all sides of the factory cell box — GridView has no spacing properties, the child margins are the only inter-cell gap mechanism (libadwaita paints gridview > child:selected over the whole cell). 3px everywhere = 6px gap on both axes, matching GNOME's 6px spacing rhythm; #478's 4/6 split produced unequal 8/12px gaps.

[Linuxuser67]

beta.16 investigation: the chip (the theme's gridview > child:selected painting) spans the whole cell node, so #478's margins on the inner box only moved content inside the chip. The 3px margins now go on the ListItem itself — the widget the chip is painted on — so the chip shrinks with the gap.

[Linuxuser67]

Correction to the first attempt: GtkListItem is not a GtkWidget (docs confirm ancestors = GObject only), so Rust-side margins don't exist and my set_margin block wouldn't compile. The chip is painted on the item's gridview > child CSS node spanning the whole cell, so the 3px gaps now ship as an app stylesheet rule on that exact node (margin: 3px, APPLICATION priority, loaded once per display) — the same node adwaita itself pads by 3px for these cells.