feat: add WidgetValueStore for centralized widget value management (#8594)

## Summary

Implements Phase 1 of the **Vue-owns-truth** pattern for widget values.
Widget values are now canonical in a Pinia store; `widget.value`
delegates to the store while preserving full backward compatibility.

## Changes

- **New store**: `src/stores/widgetValueStore.ts` - centralized widget
value storage with `get/set/remove/removeNode` API
- **BaseWidget integration**: `widget.value` getter/setter now delegates
to store when widget is associated with a node
- **LGraphNode wiring**: `addCustomWidget()` automatically calls
`widget.setNodeId(this.id)` to wire widgets to their nodes
- **Test fixes**: Added Pinia setup to test files that use widgets

## Why

This foundation enables:
- Vue components to reactively bind to widget values via `computed(() =>
store.get(...))`
- Future Yjs/CRDT backing for real-time collaboration
- Cleaner separation between Vue state and LiteGraph rendering

## Backward Compatibility

| Extension Pattern | Status |
|-------------------|--------|
| `widget.value = x` | ✅ Works unchanged |
| `node.widgets[i].value` | ✅ Works unchanged |
| `widget.callback` | ✅ Still fires |
| `node.onWidgetChanged` | ✅ Still fires |

## Testing

- ✅ 4252 unit tests pass
- ✅ Build succeeds

┆Issue is synchronized with this [Notion
page](https://www.notion.so/PR-8594-feat-add-WidgetValueStore-for-centralized-widget-value-management-2fc6d73d36508160886fcb9f3ebd941e)
by [Unito](https://www.unito.io)

---------

Co-authored-by: Amp <amp@ampcode.com>
Co-authored-by: github-actions <github-actions@github.com>
Co-authored-by: GitHub Action <action@github.com>

src/composables/graph/useGraphNodeManager.test.ts

@@ -1,49 +1,76 @@
 import { setActivePinia } from 'pinia'
 import { createTestingPinia } from '@pinia/testing'
-import { describe, expect, it, vi } from 'vitest'
-import { nextTick, watch } from 'vue'
+import { beforeEach, describe, expect, it, vi } from 'vitest'
+import { computed, nextTick, watch } from 'vue'
 
 import { useGraphNodeManager } from '@/composables/graph/useGraphNodeManager'
-import { LGraph, LGraphNode } from '@/lib/litegraph/src/litegraph'
+import { BaseWidget, LGraph, LGraphNode } from '@/lib/litegraph/src/litegraph'
 import { NodeSlotType } from '@/lib/litegraph/src/types/globalEnums'
-
-setActivePinia(createTestingPinia())
-
-function createTestGraph() {
-  const graph = new LGraph()
-  const node = new LGraphNode('test')
-  node.addInput('input', 'INT')
-  node.addWidget('number', 'testnum', 2, () => undefined, {})
-  graph.add(node)
-
-  const { vueNodeData } = useGraphNodeManager(graph)
-  const onReactivityUpdate = vi.fn()
-  watch(vueNodeData, onReactivityUpdate)
-
-  return [node, graph, onReactivityUpdate] as const
-}
+import { useWidgetValueStore } from '@/stores/widgetValueStore'
 
 describe('Node Reactivity', () => {
-  it('should trigger on callback', async () => {
-    const [node, , onReactivityUpdate] = createTestGraph()
-
-    node.widgets![0].callback!(2)
-    await nextTick()
-    expect(onReactivityUpdate).toHaveBeenCalledTimes(1)
+  beforeEach(() => {
+    setActivePinia(createTestingPinia({ stubActions: false }))
   })
 
-  it('should remain reactive after a connection is made', async () => {
-    const [node, graph, onReactivityUpdate] = createTestGraph()
+  function createTestGraph() {
+    const graph = new LGraph()
+    const node = new LGraphNode('test')
+    node.addInput('input', 'INT')
+    node.addWidget('number', 'testnum', 2, () => undefined, {})
+    graph.add(node)
+
+    const { vueNodeData } = useGraphNodeManager(graph)
+
+    return { node, graph, vueNodeData }
+  }
+
+  it('widget values are reactive through the store', async () => {
+    const { node } = createTestGraph()
+    const store = useWidgetValueStore()
+    const widget = node.widgets![0]
+
+    // Verify widget is a BaseWidget with correct value and node assignment
+    expect(widget).toBeInstanceOf(BaseWidget)
+    expect(widget.value).toBe(2)
+    expect((widget as BaseWidget).node.id).toBe(node.id)
+
+    // Initial value should be in store after setNodeId was called
+    expect(store.getWidget(node.id, 'testnum')?.value).toBe(2)
+
+    const onValueChange = vi.fn()
+    const widgetValue = computed(
+      () => store.getWidget(node.id, 'testnum')?.value
+    )
+    watch(widgetValue, onValueChange)
+
+    widget.value = 42
+    await nextTick()
+
+    expect(widgetValue.value).toBe(42)
+    expect(onValueChange).toHaveBeenCalledTimes(1)
+  })
+
+  it('widget values remain reactive after a connection is made', async () => {
+    const { node, graph } = createTestGraph()
+    const store = useWidgetValueStore()
+    const onValueChange = vi.fn()
 
     graph.trigger('node:slot-links:changed', {
-      nodeId: '1',
+      nodeId: String(node.id),
       slotType: NodeSlotType.INPUT
     })
     await nextTick()
-    onReactivityUpdate.mockClear()
 
-    node.widgets![0].callback!(2)
+    const widgetValue = computed(
+      () => store.getWidget(node.id, 'testnum')?.value
+    )
+    watch(widgetValue, onValueChange)
+
+    node.widgets![0].value = 99
     await nextTick()
-    expect(onReactivityUpdate).toHaveBeenCalledTimes(1)
+
+    expect(onValueChange).toHaveBeenCalledTimes(1)
+    expect(widgetValue.value).toBe(99)
   })
 })