[bugfix] Fix node replacements not loading due to feature flag timing (#9037)

## Summary
- Node replacements were never loaded because
`useNodeReplacementStore().load()` was called before `api.init()`,
meaning `serverFeatureFlags` was always empty at that point
- Dispatch `feature_flags` as a custom event from `api.ts` and trigger
`load()` in response within `addApiUpdateHandlers()`

## Changes
- **`api.ts`**: Dispatch `feature_flags` custom event after storing
server feature flags (already typed in `BackendApiCalls`)
- **`app.ts`**: Replace eager `load()` call with `feature_flags` event
listener inside `addApiUpdateHandlers()`, consistent with other API
event handlers
- **`nodeReplacementStore.ts`**: Use `api.getServerFeature()` directly
instead of `useFeatureFlags` composable; remove side effects from store
setup
- **`nodeReplacementStore.test.ts`**: Update mocks to match new
`api.getServerFeature` usage

## Review Focus
- Initialization ordering: listener registered before `api.init()`
ensures no missed events

┆Issue is synchronized with this [Notion
page](https://www.notion.so/PR-9037-bugfix-Fix-node-replacements-not-loading-due-to-feature-flag-timing-30e6d73d36508107ae2cd72e83c01e1a)
by [Unito](https://www.unito.io)

src/scripts/app.ts

@@ -739,6 +739,10 @@ export class ComfyApp {
       releaseSharedObjectUrl(blobUrl)
     })
 
+    api.addEventListener('feature_flags', () => {
+      void useNodeReplacementStore().load()
+    })
+
     api.init()
   }
 
@@ -802,7 +806,6 @@ export class ComfyApp {
     await useWorkspaceStore().workflow.syncWorkflows()
     //Doesn't need to block. Blueprints will load async
     void useSubgraphStore().fetchSubgraphs()
-    await useNodeReplacementStore().load()
     await useExtensionService().loadExtensions()
 
     this.addProcessKeyHandler()