Add node pinning functionality to Vue nodes (#5772)

## Summary

Added pinning functionality to Vue nodes with hotkey support and visual
indicators.

## Changes

- **What**: Added node pinning feature with 'p' hotkey toggle and pin
icon indicator
- **Components**: Updated `LGraphNode.vue` and `NodeHeader.vue` with pin
state tracking
- **State Management**: Extended `useGraphNodeManager` to sync pinned
flag with Vue components
- **Tests**: Added E2E tests for single and multi-node pin toggling

## Review Focus

Pin state persistence in graph serialization and visual indicator
positioning in node header layout. Verify hotkey doesn't conflict with
existing shortcuts.

## Technical Details

- Pin state tracked via `flags.pinned` property in `LGraphNode`
- Uses [Vue
memoization](https://vuejs.org/api/reactivity-advanced.html#v-memo) for
efficient header re-rendering
- Integrates with existing node property change detection system
- Visual indicator uses Lucide pin icon with theme-aware styling

## Screenshots (if applicable)

<img width="875" height="977" alt="Screenshot from 2025-09-25 13-02-21"
src="https://github.com/user-attachments/assets/51d46cea-08f0-44fb-8b07-56d1b939338f"
/>

<img width="875" height="977" alt="Screenshot from 2025-09-25 13-02-10"
src="https://github.com/user-attachments/assets/ce247426-1e39-48c0-924b-658b65c24f52"
/>

## Related 

- https://github.com/Comfy-Org/ComfyUI_frontend/pull/5715

┆Issue is synchronized with this [Notion
page](https://www.notion.so/PR-5772-Add-node-pinning-functionality-to-Vue-nodes-2796d73d36508195914bcfc986aa66b5)
by [Unito](https://www.unito.io)

---------

Co-authored-by: filtered <176114999+webfiltered@users.noreply.github.com>

src/lib/litegraph/src/LGraphNode.ts

@@ -3398,9 +3398,7 @@ export class LGraphNode
     this.graph._version++
     this.flags.pinned = v ?? !this.flags.pinned
     this.resizable = !this.pinned
-    // Delete the flag if unpinned, so that we don't get unnecessary
-    // flags.pinned = false in serialized object.
-    if (!this.pinned) delete this.flags.pinned
+    if (!this.pinned) this.flags.pinned = undefined
   }
 
   unpin(): void {

src/lib/litegraph/src/LGraphNodeProperties.ts

@@ -6,6 +6,7 @@ import type { LGraphNode } from './LGraphNode'
 const DEFAULT_TRACKED_PROPERTIES: string[] = [
   'title',
   'flags.collapsed',
+  'flags.pinned',
   'mode'
 ]