Context menu: trailing check mark, faded icons on disabled rows

Checked rows (Properties, grip Stretch/Copy) drew a leading "✓ " that
pushed their label out of line with the other rows. The check mark now
sits in the trailing column where submenu headers draw their caret.

Disabled Undo/Redo rows had grey text but full-strength icons:
themed_disabled faded the icon through the SVG tint colour's alpha, which
iced ignores (the tint only replaces RGB). Fade via the widget's opacity
instead, which fixes every themed_disabled icon.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01KeV3kCDBWCrpYaJEs8CRQU

src/ui/icons.rs

@@ -509,13 +509,16 @@ pub fn themed_secondary<'a, M: 'a>(bytes: &'static [u8], size: f32) -> Element<'
         .into()
 }
 
-/// Render disabled chrome with the active Iced theme's text color.
+/// Render disabled chrome with the active Iced theme's text color, faded
+/// like disabled text. The fade goes through the widget's opacity: an SVG
+/// tint only replaces RGB, so alpha on the style colour would be ignored.
 pub fn themed_disabled<'a, M: 'a>(bytes: &'static [u8], size: f32) -> Element<'a, M> {
     svg(themed_handle(bytes))
         .width(size)
         .height(size)
+        .opacity(0.42_f32)
         .style(|theme: &Theme, _| svg::Style {
-            color: Some(theme.palette().background.base.text.scale_alpha(0.42)),
+            color: Some(theme.palette().background.base.text),
         })
         .into()
 }