Implement selection state management in Vue Nodes (#5421)

* let canvas continue to own selection state management

* fix merge error

* refactor: use computed instead of watcher for selectedNodeIds

Replace watcher pattern with computed for better Vue idioms:
- More reactive and efficient
- Automatically recomputes when dependencies change
- Simpler, more declarative code

🤖 Generated with [Claude Code](https://claude.ai/code)

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

* fix: improve injection error handling for selectedNodeIds

Replace silent fallback with explicit error when SelectedNodeIds
is not provided:
- Fail fast instead of silently using empty Set
- Clear error message for debugging
- Prevents nodes appearing unselected due to missing provider

Addresses DrJKL's concern about injection default behavior.

🤖 Generated with [Claude Code](https://claude.ai/code)

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

* test: improve mocking patterns using vi.mockObject

Replace manual mock interfaces with vi.mockObject for better type safety:
- Use Vitest's built-in mocking utilities instead of manual interfaces
- Properly configure mock return values
- Remove unnecessary type assertions

Addresses DrJKL's feedback on test mocking patterns.

🤖 Generated with [Claude Code](https://claude.ai/code)

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

* test: extract repeated nodeData for clarity

Extract common test nodeData object to reduce duplication:
- Move repeated VueNodeData object to describe scope
- Replace 6 instances of identical nodeData declarations
- Maintain different nodeData for specific test cases

Addresses DrJKL's suggestion to extract repeated test data.

🤖 Generated with [Claude Code](https://claude.ai/code)

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

* add type safety to mocks

---------

Co-authored-by: Claude <noreply@anthropic.com>

src/composables/graph/useNodeEventHandlers.ts

@@ -33,12 +33,20 @@ export function useNodeEventHandlers(nodeManager: Ref<NodeManager | null>) {
     const node = nodeManager.value.getNode(nodeData.id)
     if (!node) return
 
-    // Handle multi-select with Ctrl/Cmd key
-    if (!event.ctrlKey && !event.metaKey) {
-      canvasStore.canvas.deselectAllNodes()
-    }
+    const isMultiSelect = event.ctrlKey || event.metaKey
 
-    canvasStore.canvas.selectNode(node)
+    if (isMultiSelect) {
+      // Ctrl/Cmd+click -> toggle selection
+      if (node.selected) {
+        canvasStore.canvas.deselect(node)
+      } else {
+        canvasStore.canvas.select(node)
+      }
+    } else {
+      // Regular click -> single select
+      canvasStore.canvas.deselectAll()
+      canvasStore.canvas.select(node)
+    }
 
     // Bring node to front when clicked (similar to LiteGraph behavior)
     // Skip if node is pinned to avoid unwanted movement
@@ -47,9 +55,6 @@ export function useNodeEventHandlers(nodeManager: Ref<NodeManager | null>) {
       layoutMutations.bringNodeToFront(nodeData.id)
     }
 
-    // Ensure node selection state is set
-    node.selected = true
-
     // Update canvas selection tracking
     canvasStore.updateSelectedItems()
   }