Add a colour wheel for the viewport backgrounds

The Display page could only take a typed #RRGGBB, which is precise but
no way to choose a colour by eye. The swatches are now buttons that
open a wheel.

The wheel does not replace the hex field, it sits beside it: typing an
exact value stays the quickest way to reproduce a known colour. On
submit it hands a hex string to the existing ModelSpaceBgChanged and
friends rather than writing the config itself, so the two routes
cannot drift apart on validation, persistence, or the MatchTheme
fallback an empty field triggers.

Also stops the Browser shipping in the default dock layout. It was
added to the right edge, which shifted the block palette to index 1
and broke four dock tests that encode the existing arrangement. Those
tests were right and the change was wrong: the earlier commit claimed
to leave the layout alone. BROWSER now docks the panel on first use,
so nobody's edges rearrange on upgrade for a panel they never asked
for.

src/ui/window/options.rs

@@ -146,6 +146,43 @@ impl<T> fmt::Display for Labelled<T> {
     }
 }
 
+/// A background swatch that opens the colour wheel when clicked.
+///
+/// The wheel sits alongside the hex field rather than replacing it: typing
+/// `#1E1E1E` stays the fastest way to reproduce an exact colour, while the
+/// wheel is for choosing one by eye.
+fn bg_swatch_picker<'a>(
+    target: crate::app::BgTarget,
+    rgb: [u8; 3],
+    open: Option<crate::app::BgTarget>,
+) -> Element<'a, Message> {
+    let colour = iced::Color::from_rgb8(rgb[0], rgb[1], rgb[2]);
+    let swatch = container(Space::new())
+        .width(28)
+        .height(22)
+        .style(move |theme: &Theme| container::Style {
+            background: Some(Background::Color(colour)),
+            border: Border {
+                color: theme.palette().background.strong.color,
+                width: 1.0,
+                radius: 3.0.into(),
+            },
+            ..Default::default()
+        });
+    let underlay = button(swatch)
+        .on_press(Message::BgPickerOpen(target))
+        .padding(0)
+        .style(button::text);
+    iced_aw::ColorPicker::new(
+        open == Some(target),
+        colour,
+        underlay,
+        Message::BgPickerCancel,
+        Message::BgPickerSubmit,
+    )
+    .into()
+}
+
 #[allow(clippy::too_many_arguments)]
 pub fn view_window<'a>(
     default_save_format: &'a str,
@@ -172,6 +209,7 @@ pub fn view_window<'a>(
     model_bg_input: &'a str,
     paper_bg_input: &'a str,
     desk_bg_input: &'a str,
+    bg_picker: Option<crate::app::BgTarget>,
     sizing: crate::ui::modal::ModalSizing,
 ) -> Element<'a, Message> {
     let selected_format = crate::io::SAVE_FORMAT_OPTIONS
@@ -455,58 +493,10 @@ pub fn view_window<'a>(
         });
 
     let model_bg_rgb = model_space.custom_bg.unwrap_or(crate::app::config::CLASSIC_CAD_DARK_BG);
-    let model_bg_swatch = container(Space::new())
-        .width(28)
-        .height(22)
-        .style(move |theme: &Theme| container::Style {
-            background: Some(Background::Color(iced::Color::from_rgb8(
-                model_bg_rgb[0],
-                model_bg_rgb[1],
-                model_bg_rgb[2],
-            ))),
-            border: Border {
-                color: theme.palette().background.strong.color,
-                width: 1.0,
-                radius: 3.0.into(),
-            },
-            ..Default::default()
-        });
 
     let paper_bg_rgb = model_space.custom_paper_bg.unwrap_or(crate::app::config::DEFAULT_PAPER_BG);
-    let paper_bg_swatch = container(Space::new())
-        .width(28)
-        .height(22)
-        .style(move |theme: &Theme| container::Style {
-            background: Some(Background::Color(iced::Color::from_rgb8(
-                paper_bg_rgb[0],
-                paper_bg_rgb[1],
-                paper_bg_rgb[2],
-            ))),
-            border: Border {
-                color: theme.palette().background.strong.color,
-                width: 1.0,
-                radius: 3.0.into(),
-            },
-            ..Default::default()
-        });
 
     let desk_bg_rgb = model_space.custom_desk_bg.unwrap_or(crate::app::config::DEFAULT_DESK_BG);
-    let desk_bg_swatch = container(Space::new())
-        .width(28)
-        .height(22)
-        .style(move |theme: &Theme| container::Style {
-            background: Some(Background::Color(iced::Color::from_rgb8(
-                desk_bg_rgb[0],
-                desk_bg_rgb[1],
-                desk_bg_rgb[2],
-            ))),
-            border: Border {
-                color: theme.palette().background.strong.color,
-                width: 1.0,
-                radius: 3.0.into(),
-            },
-            ..Default::default()
-        });
 
     let mode_options = crate::app::config::ModelSpaceMode::ALL
         .into_iter()
@@ -641,7 +631,7 @@ pub fn view_window<'a>(
         display = display.push(Space::new().height(10)).push(
             row![
                 text(crate::t!("Model background")).size(12).width(140),
-                model_bg_swatch,
+                bg_swatch_picker(crate::app::BgTarget::Model, model_bg_rgb, bg_picker),
                 text_input("#RRGGBB", model_bg_input)
                     .on_input(Message::ModelSpaceBgChanged)
                     .width(150),
@@ -654,7 +644,7 @@ pub fn view_window<'a>(
     display = display.push(Space::new().height(10)).push(
         row![
             text(crate::t!("Paper background")).size(12).width(140),
-            paper_bg_swatch,
+            bg_swatch_picker(crate::app::BgTarget::Paper, paper_bg_rgb, bg_picker),
             text_input("#RRGGBB", paper_bg_input)
                 .on_input(Message::PaperSpaceBgChanged)
                 .width(150),
@@ -666,7 +656,7 @@ pub fn view_window<'a>(
     display = display.push(Space::new().height(10)).push(
         row![
             text(crate::t!("Desk surround")).size(12).width(140),
-            desk_bg_swatch,
+            bg_swatch_picker(crate::app::BgTarget::Desk, desk_bg_rgb, bg_picker),
             text_input("#RRGGBB", desk_bg_input)
                 .on_input(Message::DeskSpaceBgChanged)
                 .width(150),