feat: add feature flag to disable Essentials tab in node library (#9067)

## Summary

Add a `node_library_essentials_enabled` feature flag to gate the
Essentials tab, allowing the rest of the new node library to ship while
the Essentials tab is finalized.

## Changes

- **What**: New feature flag (`node_library_essentials_enabled`) that
hides the Essentials tab in the node library sidebar and search category
sidebar. Defaults to `true` in dev/nightly builds, `false` in
production. Overridable via remote config or server feature flags. Falls
back to the "All" tab if a user previously had Essentials selected.

**Disabled UI**

<img width="547" height="782" alt="image"
src="https://github.com/user-attachments/assets/bcfcecd4-cbae-4d7b-9bcc-64bdf57929e2"
/>

**Enabled UI**

<img width="547" height="782" alt="image"
src="https://github.com/user-attachments/assets/0fb030ea-3bde-475e-982b-45e8f190cb8f"
/>


## Review Focus

- Feature flag pattern follows existing conventions (e.g.
`linearToggleEnabled`)
- Fallback behavior when essentials tab was previously selected by user

┆Issue is synchronized with this [Notion
page](https://www.notion.so/PR-9067-feat-add-feature-flag-to-disable-Essentials-tab-in-node-library-30e6d73d36508103b3cad9fc5d260611)
by [Unito](https://www.unito.io)

---------

Co-authored-by: GitHub Action <action@github.com>

src/composables/useFeatureFlags.ts

@@ -22,7 +22,8 @@ export enum ServerFeatureFlag {
   LINEAR_TOGGLE_ENABLED = 'linear_toggle_enabled',
   TEAM_WORKSPACES_ENABLED = 'team_workspaces_enabled',
   USER_SECRETS_ENABLED = 'user_secrets_enabled',
-  NODE_REPLACEMENTS = 'node_replacements'
+  NODE_REPLACEMENTS = 'node_replacements',
+  NODE_LIBRARY_ESSENTIALS_ENABLED = 'node_library_essentials_enabled'
 }
 
 /**
@@ -118,6 +119,17 @@ export function useFeatureFlags() {
     },
     get nodeReplacementsEnabled() {
       return api.getServerFeature(ServerFeatureFlag.NODE_REPLACEMENTS, false)
+    },
+    get nodeLibraryEssentialsEnabled() {
+      if (isNightly || import.meta.env.DEV) return true
+
+      return (
+        remoteConfig.value.node_library_essentials_enabled ??
+        api.getServerFeature(
+          ServerFeatureFlag.NODE_LIBRARY_ESSENTIALS_ENABLED,
+          false
+        )
+      )
     }
   })