fix: exclude missing assets from cloud mode dropdown (COM-14333) (#8747)

## Summary

Fixes a bug where non-existent images appeared in the asset search
dropdown when loading workflows that reference images the user doesn't
have in cloud mode.

## Changes

- Add `displayItems` prop to `FormDropdown` and `FormDropdownInput` for
showing selected values that aren't in the dropdown list
- Exclude `missingValueItem` from cloud asset mode `dropdownItems` while
still displaying it in the input field via `displayItems`
- Use localized error messages in `ImagePreview` for missing images
(`g.imageDoesNotExist`, `g.unknownFile`)
- Add tests for cloud asset mode behavior in
`WidgetSelectDropdown.test.ts`

## Context

The `missingValueItem` was originally added in PR #8276 for template
workflows. This fix keeps that behavior for local mode but excludes it
from cloud asset mode dropdown. Cloud users can't access files they
don't own, so showing them as search results causes confusion.

## Testing

- Added unit tests for cloud asset mode behavior
- Verified existing tests pass
- All quality gates pass: typecheck, lint, format, tests

Fixes COM-14333



┆Issue is synchronized with this [Notion
page](https://www.notion.so/PR-8747-fix-exclude-missing-assets-from-cloud-mode-dropdown-COM-14333-3016d73d365081e3ab47c326d791257e)
by [Unito](https://www.unito.io)

---------

Co-authored-by: Alexander Brown <drjkl@comfy.org>
Co-authored-by: github-actions <github-actions@github.com>

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

@@ -254,9 +254,8 @@ const baseModelFilteredAssetItems = computed<FormDropdownItem[]>(() =>
 
 const allItems = computed<FormDropdownItem[]>(() => {
   if (props.isAssetMode && assetData) {
-    if (missingValueItem.value) {
-      return [missingValueItem.value, ...baseModelFilteredAssetItems.value]
-    }
+    // Cloud assets not in user's library shouldn't appear as search results (COM-14333).
+    // Unlike local mode, cloud users can't access files they don't own.
     return baseModelFilteredAssetItems.value
   }
   return [
@@ -282,6 +281,17 @@ const dropdownItems = computed<FormDropdownItem[]>(() => {
   }
 })
 
+/**
+ * Items used for display in the input field. In cloud mode, includes
+ * missing items so users can see their selected value even if not in library.
+ */
+const displayItems = computed<FormDropdownItem[]>(() => {
+  if (props.isAssetMode && assetData && missingValueItem.value) {
+    return [missingValueItem.value, ...baseModelFilteredAssetItems.value]
+  }
+  return dropdownItems.value
+})
+
 const mediaPlaceholder = computed(() => {
   const options = props.widget.options
 
@@ -332,18 +342,20 @@ const acceptTypes = computed(() => {
 const layoutMode = ref<LayoutMode>(props.defaultLayoutMode ?? 'grid')
 
 watch(
-  [modelValue, dropdownItems],
-  ([currentValue, _dropdownItems]) => {
+  [modelValue, displayItems],
+  ([currentValue]) => {
     if (currentValue === undefined) {
       selectedSet.value.clear()
       return
     }
 
-    const item = dropdownItems.value.find((item) => item.name === currentValue)
-    if (item) {
+    const item = displayItems.value.find((item) => item.name === currentValue)
+    if (!item) {
       selectedSet.value.clear()
-      selectedSet.value.add(item.id)
+      return
     }
+    selectedSet.value.clear()
+    selectedSet.value.add(item.id)
   },
   { immediate: true }
 )
@@ -461,6 +473,7 @@ function getMediaUrl(
       v-model:ownership-selected="ownershipSelected"
       v-model:base-model-selected="baseModelSelected"
       :items="dropdownItems"
+      :display-items="displayItems"
       :placeholder="mediaPlaceholder"
       :multiple="false"
       :uploadable