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/renderer/extensions/vueNodes/components/NodeWidgets.test.ts

@@ -13,15 +13,12 @@ describe('NodeWidgets', () => {
   const createMockWidget = (
     overrides: Partial<SafeWidgetData> = {}
   ): SafeWidgetData => ({
+    nodeId: 'test_node',
     name: 'test_widget',
     type: 'combo',
-    value: 'test_value',
-    options: {
-      values: ['option1', 'option2']
-    },
+    options: undefined,
     callback: undefined,
     spec: undefined,
-    label: undefined,
     isDOMWidget: false,
     slotMetadata: undefined,
     ...overrides