feat: uniform network background; drop-target cell glows during drags

Cells and the gaps between them read as ONE surface: the dormant
uniform_background flag is on by default (the graph's bg_color carries
the cell tint and blur marker; no cell/gap quads, no grout). The only
cell highlight left is the drop target: while a node drag is in flight,
the cell it will land on (drop_target_cell_rect, cce-ui@…) wears a
faint light-pink halo — three layered expansions drawn under the node
bodies, so with grid snap the glow rings the dragged body.

Co-Authored-By: Claude Fable 5 <[email protected]>

 src/app.rs    |  5 ++++-
 src/render.rs | 15 +++++++++++++++
 2 files changed, 19 insertions(+), 1 deletion(-)

diff --git a/src/app.rs b/src/app.rs
index 5f71c6e..d5b1268 100644
--- a/src/app.rs
+++ b/src/app.rs
@@ -3293,7 +3293,10 @@ pub(crate) fn geometry_to_spreadsheet_data(geom: &Geometry) -> (Vec<String>, Vec
             last_autosave_time: std::time::Instant::now(),
             active_menu_cloud_pid: None,
             active_menu_cloud_idx: None,
-            uniform_background: false,
+            // Cells and gaps render as ONE surface (the graph's bg_color is
+            // the cell tint): the checkerboard grout is off by design; the
+            // drop-target glow (render.rs) carries the only cell highlight.
+            uniform_background: true,
             network_opacity: 0.95,
             node_opacity: 1.0,
             last_design_mod_time: {
diff --git a/src/render.rs b/src/render.rs
index a14a2f0..75492b0 100644
--- a/src/render.rs
+++ b/src/render.rs
@@ -372,6 +372,21 @@ impl State {
                         pc.quad(rect(qx, qy, qw, qh), qc);
                     }
                 }
+                // Drop-target glow: while a node drag is in flight, the cell
+                // it will land on wears a faint light-pink halo — layered
+                // expansions under the node bodies, so with grid snap on
+                // (node covering the cell exactly) the glow reads as a ring
+                // emanating around the dragged body.
+                if let Some((gx, gy, gw, gh)) = self.graph().drop_target_cell_rect() {
+                    for (grow, alpha) in [(10.0f32, 0.05f32), (5.0, 0.08), (0.0, 0.11)] {
+                        pc.rounded_rect(
+                            rect(gx - grow, gy - grow, gw + 2.0 * grow, gh + 2.0 * grow),
+                            cell_r + grow,
+                            (true, true, true, true),
+                            [1.0, 0.72, 0.80, alpha],
+                        );
+                    }
+                }
                 for (qx, qy, qw, qh, highlighted) in bodies {
                     if highlighted {
                         pc.bevel_tinted(rect(qx, qy, qw, qh), radii, node_fill, node_bevel, hl_tint);