Add node dragging, animated flow dots, and canvas cleanup to graph editor

- Drag nodes to reposition with dead-zone, edge re-routing, and minimap sync
- Animated flow dots trace upstream chains to running nodes
- Manual positions persist across re-renders, cleared on relayout
- Fix canvas event listener leak on re-render by calling destroy()

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

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

@@ -245,6 +245,15 @@
     cursor: pointer;
 }
 
+.graph-node.dragging {
+    cursor: grabbing;
+    opacity: 0.85;
+}
+
+.graph-node.dragging .graph-node-overlay {
+    display: none !important;
+}
+
 .graph-node-body {
     fill: var(--card-bg);
     stroke: var(--border-color);
@@ -393,11 +402,13 @@
 .graph-edge-flow {
     fill: none;
     stroke-width: 0;
+    pointer-events: none;
 }
 
 .graph-edge-flow circle {
     r: 3;
-    opacity: 0.8;
+    opacity: 0.85;
+    filter: drop-shadow(0 0 2px currentColor);
 }
 
 /* ── Drag connection preview ── */