Settings: controls give way, notices fold, and every edit needs its owner

A settings row's control slot shrinks to the box, so a control's text
wraps and a palette's row folds; the font size's sample wraps in a plain
block. A notice's boxes carry their channel's name and fold under the
kind's name when narrow. canvas::panel::editing takes the owner's
handler itself: the design panels, the structure editor, the index
manager and the privacy, notice and startup settings take theirs at
render and fail loud without one. Apply fails loud if pressed while the
proxy does not read. Tests cover Apply resting on a bad port, the way
back resting at the default size and a box pressed off.

src/canvas/style.rs

@@ -56,13 +56,14 @@ impl ColorPanel {
 
 impl RenderOnce for ColorPanel {
     fn render(self, _: &mut Window, cx: &mut App) -> impl IntoElement {
-        let (picked, chosen) = (self.on_change.clone(), self.on_change);
-        let report = |on_change: Option<OnEdit<Hsla>>| {
+        let on_change = self
+            .on_change
+            .unwrap_or_else(|| panic!("color panel {:?} has no on_change", self.id));
+        let (picked, chosen) = (on_change.clone(), on_change);
+        let report = |on_change: OnEdit<Hsla>| {
             move |color: Hsla, window: &mut Window, cx: &mut App| {
                 log::info!("color panel: {color:?}");
-                if let Some(on_change) = &on_change {
-                    on_change(color, window, cx);
-                }
+                on_change(color, window, cx);
             }
         };
         div()
@@ -120,6 +121,9 @@ impl ShadowEditor {
 
 impl RenderOnce for ShadowEditor {
     fn render(self, _: &mut Window, cx: &mut App) -> impl IntoElement {
+        let on_change = self
+            .on_change
+            .unwrap_or_else(|| panic!("shadow editor {:?} has no on_change", self.id));
         let lift = cx
             .theme()
             .elevation(Elevation::Floating)
@@ -132,7 +136,7 @@ impl RenderOnce for ShadowEditor {
                 color: shadow.color,
             })
             .expect("a floating elevation casts a shadow");
-        let (id, shadows, on_change) = (self.id, self.shadows, &self.on_change);
+        let (id, shadows, on_change) = (self.id, self.shadows, &on_change);
         let add = editing("shadow editor", &shadows, on_change, move |all, _: ()| {
             all.push(lift)
         });
@@ -265,7 +269,10 @@ const CORNERS: [&str; 4] = ["TL", "TR", "BR", "BL"];
 
 impl RenderOnce for BorderEditor {
     fn render(self, window: &mut Window, cx: &mut App) -> impl IntoElement {
-        let (id, border, on_change) = (self.id, self.border, &self.on_change);
+        let on_change = self
+            .on_change
+            .unwrap_or_else(|| panic!("border editor {:?} has no on_change", self.id));
+        let (id, border, on_change) = (self.id, self.border, &on_change);
         let parted = window.use_keyed_state((id.clone(), "split"), cx, |_, _| false);
         let uneven = border.radii.iter().any(|radius| *radius != border.radii[0]);
         let split = *parted.read(cx) || uneven;