feat(vue-nodes): snap link preview; connect on drop (#5780)

## Summary

Snap link preview to the nearest compatible slot while dragging in Vue
Nodes mode, and complete the connection on drop using the snapped
target. Mirrors LiteGraph’s first-compatible-slot logic for node-level
snapping and reuses the computed candidate for performance.

## Changes

- Snap preview end to compatible slot
  - slot under cursor via `data-slot-key` fast-path
  - node under cursor via `findInputByType` / `findOutputByType`
- Render path
- `slotLinkPreviewRenderer.ts` now renders to
`state.candidate.layout.position`
- Complete on drop
  - Prefer `state.candidate` (no re-hit-testing)
  - Fallbacks: DOM slot → node first-compatible → reroute
  - Disconnects moving input link when dropped on canvas

## Review Focus

- UX feel of snapping and drop completion (both directions)
- Performance on large graphs (mousemove path is O(1) with dataset +
single validation)
- Edge cases: reroutes, moving existing links, collapsed nodes

## Screenshots (if applicable)


https://github.com/user-attachments/assets/fbed0ae2-2231-473b-a05a-9aaf68e3f820

https://github.com/Comfy-Org/ComfyUI_frontend/pull/5780 (snapping) <--
https://github.com/Comfy-Org/ComfyUI_frontend/pull/5898 (drop on canvas
+ linkconnectoradapter refactor) <--
https://github.com/Comfy-Org/ComfyUI_frontend/pull/5903 (fix reroute
snapping)

┆Issue is synchronized with this [Notion
page](https://www.notion.so/PR-5780-feat-vue-nodes-snap-link-preview-connect-on-drop-27a6d73d365081d89c8cf570e2049c89)
by [Unito](https://www.unito.io)

---------

Co-authored-by: github-actions <github-actions@github.com>

src/lib/litegraph/src/LGraphCanvas.ts

@@ -5625,7 +5625,9 @@ export class LGraphCanvas
           const { link, inputNode, input } = resolved
           if (!inputNode || !input) continue
 
-          const endPos = inputNode.getInputPos(link.target_slot)
+          const endPos = LiteGraph.vueNodesMode
+            ? getSlotPosition(inputNode, link.target_slot, true)
+            : inputNode.getInputPos(link.target_slot)
 
           this.#renderAllLinkSegments(
             ctx,
@@ -5650,7 +5652,9 @@ export class LGraphCanvas
         const { link, outputNode, output } = resolved
         if (!outputNode || !output) continue
 
-        const startPos = outputNode.getOutputPos(link.origin_slot)
+        const startPos = LiteGraph.vueNodesMode
+          ? getSlotPosition(outputNode, link.origin_slot, false)
+          : outputNode.getOutputPos(link.origin_slot)
 
         this.#renderAllLinkSegments(
           ctx,