graph: cells and cell cursors share the node corner radius

Grid cells derived their rounding from the plate radius scaled by grid size
(plate_corner_radius * cell/512 * span), so cells, the node bodies sitting
on them, and the hosts' cell cursors each rounded differently — and the cell
value drifted with grid dimensions. cell_corner_radius now returns the node
radius (graph_node_corner_radius, still clamped to half the cell and gated
by the same grid-visibility checks), which unifies all three in every
consumer: the designer's cell tiles and empty-cell cursor read this value,
its node bodies and on-node cursor already used the node radius, and the
widget's internal paint (cce-files network page, cce-graph) drew nodes with
the node radius already.

 src/widget/display/graph.rs | 9 +++++++--
 1 file changed, 7 insertions(+), 2 deletions(-)

diff --git a/src/widget/display/graph.rs b/src/widget/display/graph.rs
index 3b55c99..90e8386 100644
--- a/src/widget/display/graph.rs
+++ b/src/widget/display/graph.rs
@@ -324,8 +324,13 @@ impl Graph {
             return 0.0;
         }
         let cell = self.grid_size_x.min(self.grid_size_y);
-        (crate::layout::plate_corner_radius() * (cell / 512.0) * crate::layout::corner_span_factor())
-            .min(cell / 2.0)
+        // The NODE radius, not a scaled-down plate radius: nodes, grid cells
+        // and the hosts' cell cursors (the designer's empty-cell cursor reads
+        // this) share one corner language. The old derivation
+        // (plate_corner_radius * cell/512 * span) gave cells a different,
+        // grid-size-dependent rounding that never matched the node bodies
+        // sitting on them.
+        crate::layout::graph_node_corner_radius().min(cell / 2.0)
     }
 
     /// The wires / connection preview / grid cells / axes / node bodies / toggles as plain