Make branch lines take their parent's color

A node's color now flows down the lines to its children, mind-map style:
Watering's teal colors the lines to Rain barrel and Drip line. It used to
color the line from the root down to Watering itself, which also left the
shared bus of a parent's branches in whichever child's color was drawn
last. Branches follow their parent by default, and stay dim when the
parent has no color.

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

src/view/canvas/scene.rs

@@ -190,7 +190,7 @@ impl Scene {
             let child = &self.nodes[child_index];
             let Some(Place::Child { parent, branch }) = doc.place(child.id) else { continue };
             let Some(&parent_index) = self.index.get(parent) else { continue };
-            let style = theme.branch_style(branch, child.color);
+            let style = theme.branch_style(branch, self.nodes[parent_index].color);
             let owner = LineOwner::Branch(child.id);
             self.connect(parent_index, child_index, Bend::NearStart, style, owner, None);
         }