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/app/update/mod.rs

@@ -7068,6 +7068,39 @@ impl OpenCADStudio {
                 Task::none()
             }
 
+            Message::BgPickerOpen(target) => {
+                self.bg_picker = Some(target);
+                Task::none()
+            }
+
+            Message::BgPickerCancel => {
+                self.bg_picker = None;
+                Task::none()
+            }
+
+            Message::BgPickerSubmit(color) => {
+                // Hand the result to the same handler the typed hex field
+                // uses, so the wheel and the field cannot drift apart on
+                // validation, persistence or the MatchTheme fallback.
+                let hex = crate::app::config::rgb_to_hex([
+                    (color.r * 255.0).round() as u8,
+                    (color.g * 255.0).round() as u8,
+                    (color.b * 255.0).round() as u8,
+                ]);
+                match self.bg_picker.take() {
+                    Some(crate::app::BgTarget::Model) => {
+                        Task::done(Message::ModelSpaceBgChanged(hex))
+                    }
+                    Some(crate::app::BgTarget::Paper) => {
+                        Task::done(Message::PaperSpaceBgChanged(hex))
+                    }
+                    Some(crate::app::BgTarget::Desk) => {
+                        Task::done(Message::DeskSpaceBgChanged(hex))
+                    }
+                    None => Task::none(),
+                }
+            }
+
             Message::ModelSpaceBgChanged(hex) => {
                 self.model_bg_input = hex.clone();
                 if hex.trim().is_empty() {