feat: add node replacement UI to Errors Tab (#9253)

## Summary

Adds a node replacement UI to the Errors Tab so users can swap missing
nodes with compatible alternatives directly from the error panel,
without opening a separate dialog.

## Changes

- **What**: New `SwapNodesCard` and `SwapNodeGroupRow` components render
swap groups in the Errors Tab; each group shows the missing node type,
its instances (with locate buttons), and a Replace button. Added
`useMissingNodeScan` composable to scan the graph for missing nodes and
populate `executionErrorStore`. Added `removeMissingNodesByType()` to
`executionErrorStore` so replaced nodes are pruned from the error list
reactively.

## Bug Fixes Found During Implementation

### Bug 1: Replaced nodes render as empty shells until page refresh

`replaceWithMapping()` directly mutates `_nodes[idx]`, bypassing the Vue
rendering pipeline entirely. Because the replacement node reuses the
same ID, `vueNodeData` retains the stale entry from the old placeholder
(`hasErrors: true`, empty widgets/inputs). `graph.setDirtyCanvas()` only
repaints the LiteGraph canvas and has no effect on Vue.

**Fix**: After `replaceWithMapping()`, manually call
`nodeGraph.onNodeAdded?.(newNode)` to trigger `handleNodeAdded` in
`useGraphNodeManager`, which runs `extractVueNodeData(newNode)` and
updates `vueNodeData` correctly. Also added a guard in `handleNodeAdded`
to skip `layoutStore.createNode()` when a layout for the same ID already
exists, preventing a duplicate `spatialIndex.insert()`.

### Bug 2: Missing node error list overwritten by incomplete server
response

Two compounding issues: (A) the server's `missing_node_type` error only
reports the *first* missing node — the old handler parsed this and
called `surfaceMissingNodes([singleNode])`, overwriting the full list
collected at load time. (B) `queuePrompt()` calls `clearAllErrors()`
before the API request; if the subsequent rescan used the stale
`has_errors` flag and found nothing, the missing nodes were permanently
lost.

**Fix**: Created `useMissingNodeScan.ts` which scans
`LiteGraph.registered_node_types` directly (not `has_errors`). The
`missing_node_type` catch block in `app.ts` now calls
`rescanAndSurfaceMissingNodes(this.rootGraph)` instead of parsing the
server's partial response.

## Review Focus

- `handleReplaceNode` removes the group from the store only when
`replaceNodesInPlace` returns at least one replaced node — should we
always clear, or only on full success?
- `useMissingNodeScan` re-scans on every execution-error change; confirm
no performance concerns for large graphs with many subgraphs.


## Screenshots 


https://github.com/user-attachments/assets/78310fc4-0424-4920-b369-cef60a123d50



https://github.com/user-attachments/assets/3d2fd5e1-5e85-4c20-86aa-8bf920e86987



┆Issue is synchronized with this [Notion
page](https://www.notion.so/PR-9253-feat-add-node-replacement-UI-to-Errors-Tab-3136d73d365081718d4ddfd628cb4449)
by [Unito](https://www.unito.io)

src/composables/graph/useGraphNodeManager.ts

@@ -14,6 +14,7 @@ import type {
 } from '@/lib/litegraph/src/interfaces'
 import type { IBaseWidget } from '@/lib/litegraph/src/types/widgets'
 import { useLayoutMutations } from '@/renderer/core/layout/operations/layoutMutations'
+import { layoutStore } from '@/renderer/core/layout/store/layoutStore'
 import { LayoutSource } from '@/renderer/core/layout/types'
 import type { NodeId } from '@/renderer/core/layout/types'
 import type { InputSpec } from '@/schemas/nodeDef/nodeDefSchemaV2'
@@ -442,6 +443,11 @@ export function useGraphNodeManager(graph: LGraph): GraphNodeManager {
       const nodePosition = { x: node.pos[0], y: node.pos[1] }
       const nodeSize = { width: node.size[0], height: node.size[1] }
 
+      // Skip layout creation if it already exists
+      // (e.g. in-place node replacement where the old node's layout is reused for the new node with the same ID).
+      const existingLayout = layoutStore.getNodeLayoutRef(id).value
+      if (existingLayout) return
+
       // Add node to layout store with final positions
       setSource(LayoutSource.Canvas)
       void createNode(id, {