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/components/searchbox/v2/NodeSearchCategorySidebar.vue

@@ -53,6 +53,7 @@ import NodeSearchCategoryTreeNode, {
   CATEGORY_UNSELECTED_CLASS
 } from '@/components/searchbox/v2/NodeSearchCategoryTreeNode.vue'
 import type { CategoryNode } from '@/components/searchbox/v2/NodeSearchCategoryTreeNode.vue'
+import { useFeatureFlags } from '@/composables/useFeatureFlags'
 import { nodeOrganizationService } from '@/services/nodeOrganizationService'
 import { useNodeDefStore } from '@/stores/nodeDefStore'
 import { NodeSourceType } from '@/types/nodeSource'
@@ -64,6 +65,7 @@ const selectedCategory = defineModel<string>('selectedCategory', {
 })
 
 const { t } = useI18n()
+const { flags } = useFeatureFlags()
 const nodeDefStore = useNodeDefStore()
 
 const topCategories = computed(() => [
@@ -79,7 +81,7 @@ const hasEssentialNodes = computed(() =>
 
 const sourceCategories = computed(() => {
   const categories = []
-  if (hasEssentialNodes.value) {
+  if (flags.nodeLibraryEssentialsEnabled && hasEssentialNodes.value) {
     categories.push({ id: 'essentials', label: t('g.essentials') })
   }
   categories.push({ id: 'custom', label: t('g.custom') })