fix: make top handles type=source so connections can be dragged from them

React Flow only allows initiating a drag from type=source handles.
Top handles were type=target so users could never start a connection
from the top of any node. With connectionMode=loose, source-to-source
connections are already allowed, so all handles can now be source.

frontend/src/components/canvas/nodes/BaseNode.tsx

@@ -38,7 +38,7 @@ export function BaseNode({ data, selected, icon: typeIcon }: BaseNodeProps) {
         minWidth: 140,
       }}
     >
-      <Handle type="target" position={Position.Top} className="!bg-[#30363d] !border-[#8b949e]" />
+      <Handle type="source" position={Position.Top} className="!bg-[#30363d] !border-[#8b949e]" />
 
       {/* Icon */}
       <div

frontend/src/components/canvas/nodes/ProxmoxGroupNode.tsx

@@ -80,7 +80,7 @@ export function ProxmoxGroupNode(props: NodeProps<Node<NodeData>>) {
         <div className="flex-1 relative" />
       </div>
 
-      <Handle type="target" position={Position.Top} className="!bg-[#30363d] !border-[#8b949e]" />
+      <Handle type="source" position={Position.Top} className="!bg-[#30363d] !border-[#8b949e]" />
       <Handle type="source" position={Position.Bottom} className="!bg-[#30363d] !border-[#8b949e]" />
 
       {/* Cluster handles — left/right for same-cluster links */}