fix: context menu dismisses on pane click, ISP in toolbar

Context menu now closes when clicking anywhere on the canvas via
onPaneClick prop. ISP device added as built-in toolbar item under
Internet section so it's always available without a database entry.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

frontend/src/pages/NetworkDiagrams/DiagramEditor.tsx

@@ -495,6 +495,7 @@ function DiagramEditorInner() {
               isDragOver={isDragOver}
               onNodeContextMenu={handleNodeContextMenu}
               onPaneContextMenu={handlePaneContextMenu}
+              onPaneClick={closeContextMenu}
             />
           </div>
           <AIAssistPanel