Canvas · Graphs and boards: a field hands focus back on Enter and Escape

writing::begin records the owner's focus, the mind map's or the tool
layer's, through forms::Editing::returning. The field keeps its words on
Enter's release and drops them on Escape, and both hand focus back, so
the map's arrows and the board's Backspace work at once after a write.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

src/canvas/tools.rs

@@ -355,6 +355,7 @@ impl RenderOnce for ToolLayer {
                     count: event.click_count,
                     shift: event.modifiers.shift,
                     side,
+                    back: held_focus.clone(),
                 };
                 let gesture = pressed_with(
                     &press,