fix: show clear error dialog for 403 whitelist failures (#10402)

## Summary
- When the cloud backend returns a 403 (user not whitelisted), the
frontend showed a generic "Prompt Execution Error" dialog with a cryptic
message
- Now catches 403 responses specifically and shows an "Access
Restricted" dialog with the backend's actual error message
- Adds `status` field to `PromptExecutionError` so error handlers can
distinguish HTTP status codes

## Changes
- `api.ts`: Added optional `status` to `PromptExecutionError`, pass
`res.status` from `queuePrompt`
- `app.ts`: New `else if` branch in the prompt error handler for `status
=== 403` — shows "Access Restricted" with the backend message

## Backwards compatible
- **Old backend** (`"not authorized"`): Shows "Access Restricted: not
authorized"
- **New backend**
([cloud#2941](https://github.com/Comfy-Org/cloud/pull/2941), `"your
account is not whitelisted for this feature"`): Shows "Access
Restricted: your account is not whitelisted for this feature"
- No behavior change for non-403 errors

## Related
- Backend fix: Comfy-Org/cloud#2941
- Notion: COM-16179

## Test plan
- [ ] Submit a prompt as a non-whitelisted user → should see "Access
Restricted" dialog with clear message
- [ ] Submit a prompt as a whitelisted user → no change in behavior
- [ ] Submit a prompt that fails for other reasons (missing nodes, etc.)
→ existing error handling unchanged

🤖 Generated with [Claude Code](https://claude.com/claude-code)

┆Issue is synchronized with this [Notion
page](https://www.notion.so/PR-10402-fix-show-clear-error-dialog-for-403-whitelist-failures-32c6d73d365081eb9528d7feac4e8681)
by [Unito](https://www.unito.io)

---------

Co-authored-by: Matt Miller <mattmiller@Matts-MacBook-Pro.local>
Co-authored-by: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

src/locales/en/main.json

@@ -1885,7 +1885,9 @@
     "loadWorkflowTitle": "Loading aborted due to error reloading workflow data",
     "noStackTrace": "No stacktrace available",
     "extensionFileHint": "This may be due to the following script",
-    "promptExecutionError": "Prompt execution failed"
+    "promptExecutionError": "Prompt execution failed",
+    "accessRestrictedTitle": "Access Restricted",
+    "accessRestrictedMessage": "Your account is not authorized for this feature."
   },
   "apiNodesSignInDialog": {
     "title": "Sign In Required to Use API Nodes",