feat: gate node replacement loading on server feature flag (#8750)

## Summary

Gates the node replacement store's `load()` call behind the
`node_replacements` server feature flag, so the frontend only calls
`/api/node_replacements` when the backend advertises support.

## Changes

- Added `NODE_REPLACEMENTS = 'node_replacements'` to `ServerFeatureFlag`
enum
- Added `nodeReplacementsEnabled` getter to `useFeatureFlags()`
- Added `api.serverSupportsFeature('node_replacements')` guard in
`useNodeReplacementStore.load()`

## Context

Without this guard, the frontend would attempt to fetch node
replacements from backends that don't support the endpoint, causing 404
errors.

Companion backend PR: https://github.com/Comfy-Org/ComfyUI/pull/12362

┆Issue is synchronized with this [Notion
page](https://www.notion.so/PR-8750-feat-gate-node-replacement-loading-on-server-feature-flag-3026d73d365081ec9246d77ad88f5bdc)
by [Unito](https://www.unito.io)

---------

Co-authored-by: Jin Yi <jin12cc@gmail.com>
Co-authored-by: Alexander Brown <drjkl@comfy.org>
Co-authored-by: Claude <noreply@anthropic.com>

src/composables/useFeatureFlags.ts

@@ -20,7 +20,8 @@ export enum ServerFeatureFlag {
   ONBOARDING_SURVEY_ENABLED = 'onboarding_survey_enabled',
   LINEAR_TOGGLE_ENABLED = 'linear_toggle_enabled',
   TEAM_WORKSPACES_ENABLED = 'team_workspaces_enabled',
-  USER_SECRETS_ENABLED = 'user_secrets_enabled'
+  USER_SECRETS_ENABLED = 'user_secrets_enabled',
+  NODE_REPLACEMENTS = 'node_replacements'
 }
 
 /**
@@ -96,6 +97,9 @@ export function useFeatureFlags() {
         remoteConfig.value.user_secrets_enabled ??
         api.getServerFeature(ServerFeatureFlag.USER_SECRETS_ENABLED, false)
       )
+    },
+    get nodeReplacementsEnabled() {
+      return api.getServerFeature(ServerFeatureFlag.NODE_REPLACEMENTS, false)
     }
   })