feat(selection): highlight opacity and extra width (#1823)

Options > Selection gains "Selection highlight opacity" and "Selection
highlight extra width". Below full opacity the object's own colour
shows through the highlight. The selection overlay draws with its own
copy of the frame uniforms: transparency on, and every line widened by
the extra width. Drawings opened at startup now get the selection
colour settings too; they used to keep the defaults until a setting
changed.

Co-Authored-By: Claude Opus 5.5 <[email protected]>

src/ui/window/options.rs

@@ -1120,6 +1120,38 @@ pub fn view_window<'a>(
         ]
         .spacing(10)
         .align_y(iced::Center),
+        Space::new().height(10),
+        row![
+            text(crate::t!("Selection highlight opacity")).size(12).width(140),
+            slider(
+                10..=100,
+                model_space.selection_highlight_opacity.clamp(10, 100),
+                Message::SelectionHighlightOpacityChanged,
+            )
+            .step(5)
+            .width(Fill),
+            text(format!("{}%", model_space.selection_highlight_opacity.clamp(10, 100)))
+                .size(11)
+                .width(44),
+        ]
+        .spacing(10)
+        .align_y(iced::Center),
+        Space::new().height(10),
+        row![
+            text(crate::t!("Selection highlight extra width")).size(12).width(140),
+            slider(
+                0..=6,
+                model_space.selection_highlight_widen.min(6),
+                Message::SelectionHighlightWidenChanged,
+            )
+            .step(1)
+            .width(Fill),
+            text(format!("{} px", model_space.selection_highlight_widen.min(6)))
+                .size(11)
+                .width(44),
+        ]
+        .spacing(10)
+        .align_y(iced::Center),
         Space::new().height(24),
         text(crate::t!("Preview")).size(15),
         Space::new().height(10),