[feat] Surface missing models in Errors tab (Cloud) (#9743)

## Summary
When a workflow is loaded with missing models, users currently have no
way to identify or resolve them from within the UI. This PR adds a full
missing-model detection and resolution pipeline that surfaces missing
models in the Errors tab, allowing users to install or import them
without leaving the editor.

## Changes

### Missing Model Detection
- Scan all COMBO widgets across root graph and subgraphs for model-like
filenames during workflow load
- Enrich candidates with embedded workflow metadata (url, hash,
directory) when available
- Verify asset-supported candidates against the asset store
asynchronously to confirm installation status
- Propagate missing model state to `executionErrorStore` alongside
existing node/prompt errors

### Errors Tab UI — Model Resolution
- Group missing models by directory (e.g. `checkpoints`, `loras`, `vae`)
with collapsible category cards
- Each model row displays:
  - Model name with copy-to-clipboard button
  - Expandable list of referencing nodes with locate-on-canvas button
- **Library selector**: Pick an alternative from the user's existing
models to substitute the missing model with one click
- **URL import**: Paste a Civitai or HuggingFace URL to import a model
directly; debounced metadata fetch shows filename and file size before
confirming; type-mismatch warnings (e.g. importing a LoRA into
checkpoints directory) are surfaced with an "Import Anyway" option
- **Upgrade prompt**: In cloud environment, free-tier subscribers are
shown an upgrade modal when attempting URL import
- Separate "Import Not Supported" section for custom-node models that
cannot be auto-resolved
- Status card with live download progress, completion, failure, and
category-mismatch states

### Canvas Integration
- Highlight nodes and widgets that reference missing models with error
indicators
- Propagate missing-model badges through subgraph containers so issues
are visible at every graph level

### Code Cleanup
- Simplify `surfacePendingWarnings` in workflowService, remove stale
widget-detected model merging logic
- Add `flattenWorkflowNodes` utility to workflowSchema for traversing
nested subgraph structures
- Extract `MissingModelUrlInput`, `MissingModelLibrarySelect`,
`MissingModelStatusCard` as focused single-responsibility components

## Testing
- Unit tests for scan pipeline (`missingModelScan.test.ts`): enrichment,
skip-installed, subgraph flattening
- Unit tests for store (`missingModelStore.test.ts`): state management,
removal helpers
- Unit tests for interactions (`useMissingModelInteractions.test.ts`):
combo select, URL input, import flow, library confirm
- Component tests for `MissingModelCard` and error grouping
(`useErrorGroups.test.ts`)
- Updated `workflowService.test.ts` and `workflowSchema.test.ts` for new
logic

## Review Focus
- Missing model scan + enrichment pipeline in `missingModelScan.ts`
- Interaction composable `useMissingModelInteractions.ts` — URL metadata
fetch, library install, upload fallback
- Store integration and canvas-level error propagation

## Screenshots 


https://github.com/user-attachments/assets/339a6d5b-93a3-43cd-98dd-0fb00681b66f



┆Issue is synchronized with this [Notion
page](https://www.notion.so/PR-9743-feat-Surface-missing-models-in-Errors-tab-Cloud-3206d73d365081678326d3a16c2165d8)
by [Unito](https://www.unito.io)

src/platform/missingModel/components/MissingModelLibrarySelect.vue

@@ -0,0 +1,113 @@
+<template>
+  <div class="flex flex-col gap-2">
+    <div v-if="showDivider" class="flex items-center justify-center py-0.5">
+      <span class="text-xs font-bold text-muted-foreground">
+        {{ t('rightSidePanel.missingModels.or') }}
+      </span>
+    </div>
+
+    <Select
+      :model-value="modelValue"
+      :disabled="options.length === 0"
+      @update:model-value="handleSelect"
+    >
+      <SelectTrigger
+        size="md"
+        class="border-transparent bg-secondary-background text-xs hover:border-interface-stroke"
+      >
+        <SelectValue
+          :placeholder="t('rightSidePanel.missingModels.useFromLibrary')"
+        />
+      </SelectTrigger>
+
+      <SelectContent>
+        <template v-if="options.length > 4" #prepend>
+          <div class="px-1 pb-1.5">
+            <div
+              class="flex items-center gap-1.5 rounded-md border border-border-default px-2"
+            >
+              <i
+                aria-hidden="true"
+                class="icon-[lucide--search] size-3.5 shrink-0 text-muted-foreground"
+              />
+              <input
+                v-model="filterQuery"
+                type="text"
+                :aria-label="t('g.searchPlaceholder', { subject: '' })"
+                class="h-7 w-full border-none bg-transparent text-xs outline-none placeholder:text-muted-foreground"
+                :placeholder="t('g.searchPlaceholder', { subject: '' })"
+                @keydown.stop
+              />
+            </div>
+          </div>
+        </template>
+
+        <SelectItem
+          v-for="option in filteredOptions"
+          :key="option.value"
+          :value="option.value"
+          class="text-xs"
+        >
+          {{ option.name }}
+        </SelectItem>
+        <div
+          v-if="filteredOptions.length === 0"
+          role="status"
+          class="px-3 py-2 text-xs text-muted-foreground"
+        >
+          {{ t('g.noResultsFound') }}
+        </div>
+      </SelectContent>
+    </Select>
+  </div>
+</template>
+
+<script setup lang="ts">
+import { computed, ref, watch } from 'vue'
+import { useI18n } from 'vue-i18n'
+import { useFuse } from '@vueuse/integrations/useFuse'
+import Select from '@/components/ui/select/Select.vue'
+import SelectContent from '@/components/ui/select/SelectContent.vue'
+import SelectTrigger from '@/components/ui/select/SelectTrigger.vue'
+import SelectValue from '@/components/ui/select/SelectValue.vue'
+import SelectItem from '@/components/ui/select/SelectItem.vue'
+
+const { options, showDivider = false } = defineProps<{
+  modelValue: string | undefined
+  options: { name: string; value: string }[]
+  showDivider?: boolean
+}>()
+
+const emit = defineEmits<{
+  select: [value: string]
+}>()
+
+const { t } = useI18n()
+
+const filterQuery = ref('')
+
+watch(
+  () => options.length,
+  (len) => {
+    if (len <= 4) filterQuery.value = ''
+  }
+)
+
+const { results: fuseResults } = useFuse(filterQuery, () => options, {
+  fuseOptions: {
+    keys: ['name'],
+    threshold: 0.4,
+    ignoreLocation: true
+  },
+  matchAllWhenSearchEmpty: true
+})
+
+const filteredOptions = computed(() => fuseResults.value.map((r) => r.item))
+
+function handleSelect(value: unknown) {
+  if (typeof value === 'string') {
+    filterQuery.value = ''
+    emit('select', value)
+  }
+}
+</script>