[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/locales/en/main.json

@@ -1957,6 +1957,7 @@
     "exportSuccess": "Successfully exported model as {format}",
     "fileLoadError": "Unable to find workflow in {fileName}",
     "dropFileError": "Unable to process dropped item: {error}",
+    "missingModelVerificationFailed": "Failed to verify missing models. Some models may not be shown in the Errors tab.",
     "interrupted": "Execution has been interrupted",
     "pendingTasksDeleted": "Pending tasks deleted",
     "nothingToGroup": "Nothing to group",
@@ -3388,6 +3389,33 @@
       "viewInManager": "View in Manager",
       "collapse": "Collapse",
       "expand": "Expand"
+    },
+    "missingModels": {
+      "urlPlaceholder": "Paste Model URL (Civitai or Hugging Face)",
+      "or": "OR",
+      "useFromLibrary": "Use from Library",
+      "usingFromLibrary": "Using from Library",
+      "unsupportedUrl": "Only Civitai and Hugging Face URLs are supported.",
+      "metadataFetchFailed": "Failed to retrieve metadata. Please check the link and try again.",
+      "import": "Import",
+      "importing": "Importing...",
+      "imported": "Imported",
+      "importFailed": "Import failed",
+      "typeMismatch": "This model seems to be a \"{detectedType}\". Are you sure?",
+      "importAnyway": "Import Anyway",
+      "alreadyExistsInCategory": "This model already exists in \"{category}\"",
+      "customNodeDownloadDisabled": "Cloud environment does not support model imports for custom nodes in this section. Please use standard loader nodes or substitute with a model from the library below.",
+      "importNotSupported": "Import Not Supported",
+      "copyModelName": "Copy model name",
+      "confirmSelection": "Confirm selection",
+      "locateNode": "Locate node on canvas",
+      "cancelSelection": "Cancel selection",
+      "clearUrl": "Clear URL",
+      "expandNodes": "Show referencing nodes",
+      "collapseNodes": "Hide referencing nodes",
+      "unknownCategory": "Unknown",
+      "missingModelsTitle": "Missing Models",
+      "assetLoadTimeout": "Model detection timed out. Try reloading the workflow."
     }
   },
   "errorOverlay": {