feat(canvas): edge endpoint reconnect + proxmox container snap points

- Selected edge shows interactive dots at both endpoints; drag onto any
  handle (different snap point or different node) to reconnect. Edge
  follows the cursor live during drag; drop on empty space leaves edge
  unchanged.
- Endpoint dots render in EdgeLabelRenderer above the node layer and walk
  the elementsFromPoint stack so the underlying handle wins over the
  dragged dot itself.
- ProxmoxGroupNode in container mode now renders bottom_handles snap
  points (was always single 'bottom' handle), matching BaseNode.
- New canvasStore.reconnectEdge action: swaps source/target/handles via
  normalizeHandle, snapshots history, marks dirty.

frontend/src/index.css

@@ -11,6 +11,13 @@
   to   { stroke-dashoffset: 0; }
 }
 
+/* Disable React Flow's built-in edgeupdater entirely — HomelableEdge renders
+   its own interactive endpoint dots in EdgeLabelRenderer (above the node
+   layer) so the node Handle DOM cannot steal the reconnection drag. */
+.react-flow__edgeupdater {
+  display: none;
+}
+
 /* Homelable dark theme — always dark */
 :root {
     --background: #0d1117;