fix: call checkState after image input changes for proper undo tracking (#9623)

## Summary

Image input changes (dropdown selection and file upload) in app/linear
mode did not create their own undo entries, causing undo to skip or
bundle image changes with subsequent actions.

## Changes

- **What**: Add explicit `checkState()` calls in
`WidgetSelectDropdown.vue` after `modelValue` is set in
`updateSelectedItems()` (dropdown selection) and `handleFilesUpdate()`
(file upload), ensuring each image change gets its own undo entry.

## Review Focus

The fix is intentionally scoped to `WidgetSelectDropdown` rather than
the generic `updateHandler` in `NodeWidgets.vue`, which would create
excessive undo entries for text inputs. The pattern follows existing
usage in `useSelectedNodeActions.ts` and other composables.

┆Issue is synchronized with this [Notion
page](https://www.notion.so/PR-9623-fix-call-checkState-after-image-input-changes-for-proper-undo-tracking-31d6d73d3650814781dbca5db459ab6d)
by [Unito](https://www.unito.io)

---------

Co-authored-by: GitHub Action <action@github.com>

src/renderer/extensions/vueNodes/widgets/components/WidgetSelectDropdown.vue

@@ -17,6 +17,7 @@ import {
   getAssetFilename
 } from '@/platform/assets/utils/assetMetadataUtils'
 import { useToastStore } from '@/platform/updates/common/toastStore'
+import { useWorkflowStore } from '@/platform/workflow/management/stores/workflowStore'
 import FormDropdown from '@/renderer/extensions/vueNodes/widgets/components/form/dropdown/FormDropdown.vue'
 import type {
   FilterOption,
@@ -376,6 +377,7 @@ function updateSelectedItems(selectedItems: Set<string>) {
     return
   }
   modelValue.value = name
+  useWorkflowStore().activeWorkflow?.changeTracker?.checkState()
 }
 
 const uploadFile = async (
@@ -450,6 +452,9 @@ async function handleFilesUpdate(files: File[]) {
     if (props.widget.callback) {
       props.widget.callback(uploadedPaths[0])
     }
+
+    // 5. Snapshot undo state so the image change gets its own undo entry
+    useWorkflowStore().activeWorkflow?.changeTracker?.checkState()
   } catch (error) {
     console.error('Upload error:', error)
     toastStore.addAlert(`Upload failed: ${error}`)