fix: link dragging offset on external monitors in Vue nodes mode (#8809)

## Summary

Fix link dragging offset when using Vue nodes mode on external monitors
with different DPI than the primary display.

## Changes

- **What**: Derive overlay canvas scale from actual canvas dimensions
(`canvas.width / canvas.clientWidth`) instead of
`window.devicePixelRatio`, fixing DPR mismatch. Map `LinkDirection.NONE`
to `'none'` in `convertDirection()` instead of falling through to
`'right'`.

## Before


https://github.com/user-attachments/assets/f5d04617-369f-4649-af60-11d31e27a75c



## After


https://github.com/user-attachments/assets/76434d2b-d485-43de-94f6-202a91f73edf


## Review Focus

The overlay canvas copies dimensions from the main canvas (which
includes DPR scaling from `resizeCanvas`). When the page loads on a
monitor whose DPR differs from what `resizeCanvas` used,
`window.devicePixelRatio` no longer matches the canvas's internal-to-CSS
ratio, causing all drawn link positions to be offset. The fix derives
scale directly from the canvas itself.

`LinkDirection.NONE = 0` is falsy, so it was caught by the `default`
case in `convertDirection()`, adding an unwanted directional curve to
moved input links.

┆Issue is synchronized with this [Notion
page](https://www.notion.so/PR-8809-fix-link-dragging-offset-on-external-monitors-in-Vue-nodes-mode-3046d73d36508143b600f23f5fe07044)
by [Unito](https://www.unito.io)

src/lib/litegraph/src/LGraphCanvas.ts

@@ -5060,7 +5060,7 @@ export class LGraphCanvas implements CustomEventDispatcher<LGraphCanvasEventMap>
 
     octx.save()
 
-    const scale = window.devicePixelRatio
+    const scale = overlayCanvas.width / (overlayCanvas.clientWidth || 1)
     octx.setTransform(scale, 0, 0, scale, 0, 0)
 
     this.ds.toCanvasContext(octx)

src/renderer/core/canvas/litegraph/litegraphLinkAdapter.ts

@@ -68,6 +68,7 @@ export class LitegraphLinkAdapter {
       case LinkDirection.DOWN:
         return 'down'
       case LinkDirection.CENTER:
+      case LinkDirection.NONE:
         return 'none'
       default:
         return 'right'