[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/MissingModelCard.vue

@@ -0,0 +1,78 @@
+<template>
+  <div class="px-4 pb-2">
+    <!-- Category groups (by directory) -->
+    <div
+      v-for="group in missingModelGroups"
+      :key="`${group.isAssetSupported ? 'supported' : 'unsupported'}::${group.directory ?? '__unknown__'}`"
+      class="flex w-full flex-col border-t border-interface-stroke py-2 first:border-t-0 first:pt-0"
+    >
+      <!-- Category header -->
+      <div class="flex h-8 w-full items-center">
+        <p
+          class="min-w-0 flex-1 truncate text-sm font-medium"
+          :class="
+            !group.isAssetSupported || group.directory === null
+              ? 'text-warning-background'
+              : 'text-destructive-background-hover'
+          "
+        >
+          <span v-if="!group.isAssetSupported" class="text-warning-background">
+            {{ t('rightSidePanel.missingModels.importNotSupported') }}
+            ({{ group.models.length }})
+          </span>
+          <span v-else>
+            {{
+              group.directory ??
+              t('rightSidePanel.missingModels.unknownCategory')
+            }}
+            ({{ group.models.length }})
+          </span>
+        </p>
+      </div>
+
+      <!-- Asset unsupported group notice -->
+      <div
+        v-if="!group.isAssetSupported"
+        class="flex items-start gap-1.5 px-0.5 py-1 pl-2"
+      >
+        <i
+          aria-hidden="true"
+          class="mt-0.5 icon-[lucide--info] size-3.5 shrink-0 text-muted-foreground"
+        />
+        <span class="text-xs/tight text-muted-foreground">
+          {{ t('rightSidePanel.missingModels.customNodeDownloadDisabled') }}
+        </span>
+      </div>
+
+      <!-- Model rows -->
+      <div class="flex flex-col gap-1 overflow-hidden pl-2">
+        <MissingModelRow
+          v-for="model in group.models"
+          :key="model.name"
+          :model="model"
+          :directory="group.directory"
+          :show-node-id-badge="showNodeIdBadge"
+          :is-asset-supported="group.isAssetSupported"
+          @locate-model="emit('locateModel', $event)"
+        />
+      </div>
+    </div>
+  </div>
+</template>
+
+<script setup lang="ts">
+import { useI18n } from 'vue-i18n'
+import type { MissingModelGroup } from '@/platform/missingModel/types'
+import MissingModelRow from '@/platform/missingModel/components/MissingModelRow.vue'
+
+const { missingModelGroups, showNodeIdBadge } = defineProps<{
+  missingModelGroups: MissingModelGroup[]
+  showNodeIdBadge: boolean
+}>()
+
+const emit = defineEmits<{
+  locateModel: [nodeId: string]
+}>()
+
+const { t } = useI18n()
+</script>