Add rubber-band selection, multi-node drag, edge click, and keyboard shortcuts

- Shift+drag on empty space draws selection rectangle to select multiple nodes
- Multi-node drag: dragging a selected node moves all selected nodes together
- Click edge to highlight it and its connected nodes
- Delete key removes single selected node, Ctrl+A selects all
- Edges now have pointer cursor for click affordance

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

server/src/wled_controller/static/css/graph-editor.css

@@ -365,6 +365,7 @@
     fill: none;
     stroke-width: 2;
     opacity: 0.6;
+    cursor: pointer;
     transition: opacity 0.15s, stroke-width 0.15s;
 }
 
@@ -373,6 +374,11 @@
     stroke-width: 3;
 }
 
+/* Wider invisible hit area for thin edges */
+.graph-edge {
+    stroke-linecap: round;
+}
+
 .graph-edge-arrow {
     fill: currentColor;
     opacity: 0.6;