Add a 2px gap around each leaf slot so per-pane borders don't merge

With panes packed edge-to-edge, adjacent .leaf elements' 2px borders
touched directly. When all (or many) panes went idle, every pane's
red border combined with its neighbour's to form continuous red lines
across the whole window — looking like a single grid pattern, not
distinct per-pane outlines.

Fix: padding: 2px on .leaf-slot (box-sizing: border-box). Each .leaf
ends up inset 2px on every side, so adjacent panes have a 4px dark
gap between them and their borders read as separate rectangles.
Affects all borders equally (idle red, active blue, broadcasting
orange) and gives the layout a cleaner separation overall.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

src/App.css

@@ -79,3 +79,11 @@
   min-height: 0;
   position: relative;
 }
+
+/* 2px padding on each leaf slot creates a 4px gap between adjacent panes —
+   so per-pane borders (idle red, active blue, broadcasting orange) read as
+   distinct rectangles instead of merging into a continuous grid pattern. */
+.leaf-slot {
+  padding: 2px;
+  box-sizing: border-box;
+}