Refactor: popups share overlayStyle and overlayEnter

internal.PopIn is gone. Breadcrumb, NavigationMenu, Pagination, Menubar
panels, CommandPalette, Select (and the other selectPanel users),
datetime popups, ColorPicker and the StatusBar overflow now style their
panel with overlayStyle(c, panel, overlayEnter(c, panel)): one shadow
(0 6 18, 16% black, scaled by entry) and one 160 ms entry fade.

Snapshots: color_picker and command_palette (light and dark) change only
in the panel shadow. ColorPicker loses its 0 4 12 18% shadow for the
softer shared one; CommandPalette loses its heavy 0 12 32 30% shadow,
and its backdrop now fades by alpha with the panel instead of through
the backdrop's opacity. Fade and reduced-motion tests pass unchanged.

pagination.go

@@ -175,9 +175,8 @@ func paginationSizes(c *ui.Context, page, pageSize *int, sizes []int, res *Choic
 		ui.Icon(c, icons.Must("chevron-down")).Size(14, 14).TextColor(k.TextMuted)
 	})
 	sel.Popup(func(panel *ui.Element) {
-		internal.PopIn(c, panel, Motion(c, theme.PopupDuration))
-		panel.Margin(u, 0, 0, 0).Padding(u).Radius(theme.PopupRadius).Background(k.Surface).
-			Border(theme.BorderWidth, k.Border).Shadow(0, 4, 12, 0, ui.RGBA(0, 0, 0, 0.16))
+		overlayStyle(c, panel, overlayEnter(c, panel))
+		panel.Margin(u, 0, 0, 0).Padding(u)
 		for _, s := range sizes {
 			it := sel.Item(s).Padding(u*1.5, u*3).Radius(theme.ControlRadius).Gap(u * 2)
 			if it.Highlighted() {