feat(persistence): fix QuotaExceededError and cross-workspace draft leakage (#8520)

## Summary

Completes the workflow persistence overhaul by integrating the new draft
system into the app and migrating existing data. Fixes two critical
bugs:

1. **QuotaExceededError** - localStorage fills up with workflow drafts,
breaking auto-save
2. **Cross-workspace data leakage** - Drafts from one ComfyUI instance
appear in another

## Changes

- **What**: 
- `useWorkflowPersistenceV2.ts` - Main composable that hooks into graph
changes with 512ms debounce
- `migrateV1toV2.ts` - One-time migration of existing drafts to the new
scoped format
  - Updated E2E tests for new storage key patterns
- **Why**: Users lose work when storage quota is exceeded, and see
confusing workflows from other instances

## How It Works

- **Workspace scoping**: Each ComfyUI instance (identified by server
URL) has isolated draft storage
- **LRU eviction**: When storage is full, oldest drafts are
automatically removed (keeps 32 most recent)
- **Tab isolation**: Each browser tab tracks its own active/open
workflows via sessionStorage
- **Debounced saves**: Graph changes are batched with 512ms delay to
reduce storage writes

## Migration

Existing V1 drafts are automatically migrated on first load. The
migration:
1. Reads drafts from old `Comfy.Workflow.*` keys
2. Converts to new workspace-scoped format
3. Cleans up old keys after successful migration

---
*Part 4 of 4 in the workflow persistence improvements stack*

---------

Co-authored-by: Amp <amp@ampcode.com>
Co-authored-by: GitHub Action <action@github.com>
Co-authored-by: Simula_r <18093452+simula-r@users.noreply.github.com>

src/composables/auth/useFirebaseAuthActions.ts

@@ -9,6 +9,7 @@ import { t } from '@/i18n'
 import { isCloud } from '@/platform/distribution/types'
 import { useTelemetry } from '@/platform/telemetry'
 import { useToastStore } from '@/platform/updates/common/toastStore'
+import { useWorkflowStore } from '@/platform/workflow/management/stores/workflowStore'
 import { useDialogService } from '@/services/dialogService'
 import { useFirebaseAuthStore } from '@/stores/firebaseAuthStore'
 import type { BillingPortalTargetTier } from '@/stores/firebaseAuthStore'
@@ -51,6 +52,17 @@ export const useFirebaseAuthActions = () => {
   }
 
   const logout = wrapWithErrorHandlingAsync(async () => {
+    const workflowStore = useWorkflowStore()
+    if (workflowStore.modifiedWorkflows.length > 0) {
+      const dialogService = useDialogService()
+      const confirmed = await dialogService.confirm({
+        title: t('auth.signOut.unsavedChangesTitle'),
+        message: t('auth.signOut.unsavedChangesMessage'),
+        type: 'dirtyClose'
+      })
+      if (!confirmed) return
+    }
+
     await authStore.logout()
     toastStore.add({
       severity: 'success',