list_item: trailing icon slot and horizontal inset override

Disclosure arrows in list rows could only be text: the trailing slot draws a string, so apps fell back to the "›" glyph at 14 px, which cannot use the theme's arrow SVGs and looks thin next to 24 px leading icons. Add ListItem::trailing_icon( rgba, w, h ): a right-aligned icon drawn at the new theme::TRAILING_ICON_SIZE (21 px, 1.5x the old glyph em box), vertically centered. It coexists with trailing text, which shifts to the icon's left. Like the leading icon, symbolic assets are pre-tinted by the caller (tint_symbolic), so the widget stays colour-agnostic.
Add ListItem::pad_h( impl Into<Length> ), a per-item override of the horizontal inset between the row edge and its content (leading icon/label on the left, trailing text/icon on the right). Resolution follows the Separator pattern: an explicit Length wins, otherwise the theme default (16 px through geom_px) applies, so existing rows are unaffected. This lets an app whose enclosing view already provides the margin bring row content flush instead of stacking both insets.
Document both builders in docs/widgets.md and the changelog, and rework the list_item rustdoc example to point at trailing_icon with a theme icon for disclosure arrows instead of the text glyph.

docs/widgets.md

@@ -144,8 +144,9 @@ each mode supplies.
 
 ### `list_item`
 
-A row with a primary label, optional subtitle, optional right-aligned
-trailing text, and a tappable surface.
+A row with a primary label, optional subtitle, optional leading icon,
+optional right-aligned trailing text and/or trailing icon, and a
+tappable surface.
 
 **When**: settings menus, navigation lists, contact rows. Doubles its
 height when a subtitle is set.
@@ -154,13 +155,27 @@ height when a subtitle is set.
 # use ltk::{ list_item, ListItem };
 # #[ derive( Clone ) ] enum Msg { OpenWifi }
 # fn _ex() -> ListItem<Msg> {
-list_item( "Wi-Fi" )
+let mut it = list_item( "Wi-Fi" )
     .subtitle( "Eduroam" )
-    .trailing( "›" )
-    .on_press( Msg::OpenWifi )
+    .on_press( Msg::OpenWifi );
+// Disclosure arrow from the active theme, tinted to match the
+// trailing-text colour.
+if let Some( ( rgba, w, h ) ) = ltk::theme_icon_rgba( "general/right", 21 )
+{
+    let tinted = std::sync::Arc::new( ltk::tint_symbolic( &rgba, ltk::theme_palette().text_secondary ) );
+    it = it.trailing_icon( tinted, w, h );
+}
+it
 # }
 ```
 
+`trailing( text )` right-aligns a text value (current setting, badge
+count); `trailing_icon( rgba, w, h )` draws an icon at the right edge.
+When both are set the text sits to the left of the icon.
+`pad_h( px )` overrides the horizontal inset between the row edge and
+its content (theme default 16 px) — lower it when the enclosing view's
+own padding already provides the margin.
+
 **See also**: [`pressable`](#pressable) for free-form tappable rows,
 [`scroll`](#scroll) to wrap a list of items in a scrollable container.