feat: unify sidebar panel header layout with SidebarTopArea component (#9740)

## Summary

Unify the search bar + action buttons layout across all left sidebar
panels (Node Library, Workflows, Model Library, Media Assets) using a
shared `SidebarTopArea` presentation component.

## Changes

- **What**:
- Add `SidebarTopArea.vue` — layout component with `flex-1` default slot
(search) and `#actions` slot (buttons), plus optional `bottomDivider`
prop
- Replace raw `<button>` elements in Node Library with `<Button
variant="secondary" size="icon">`
- Replace reka-ui `TabsTrigger` with shared `Tab/TabList` component in
Node Library
- Move Media Assets tab list from hover-only `#tool-buttons` to
always-visible header below search area
- Unify spacing (`gap-2`, `p-2 2xl:px-4`) and divider styles across all
sidebar panels
- Remove unused `assetType` prop and header from
`AssetsSidebarGridView`/`AssetsSidebarListView`

## Review Focus

- `SidebarTopArea` API simplicity — just slots + one optional prop
- Node Library still requires `TabsRoot` in the body for reka-ui
`TabsContent` in child panels
- Media Assets tabs are now always visible instead of hover-only

[screen-capture
(1).webm](https://github.com/user-attachments/assets/fe1d8f7b-5674-4bb3-9842-569e4c3af6c9)

┆Issue is synchronized with this [Notion
page](https://www.notion.so/PR-9740-feat-unify-sidebar-panel-header-layout-with-SidebarTopArea-component-3206d73d365081ea8ba7fd6ac54e0169)
by [Unito](https://www.unito.io)

---------

Co-authored-by: Amp <amp@ampcode.com>

src/components/sidebar/tabs/NodeLibrarySidebarTabV2.test.ts

@@ -1,6 +1,5 @@
 import { mount } from '@vue/test-utils'
 import { createTestingPinia } from '@pinia/testing'
-import { TabsContent, TabsList, TabsRoot, TabsTrigger } from 'reka-ui'
 import { ref } from 'vue'
 import { beforeEach, describe, expect, it, vi } from 'vitest'
 import { createI18n } from 'vue-i18n'
@@ -96,12 +95,6 @@ describe('NodeLibrarySidebarTabV2', () => {
     return mount(NodeLibrarySidebarTabV2, {
       global: {
         plugins: [createTestingPinia({ stubActions: false }), i18n],
-        components: {
-          TabsRoot,
-          TabsList,
-          TabsTrigger,
-          TabsContent
-        },
         stubs: {
           teleport: true
         }
@@ -112,7 +105,7 @@ describe('NodeLibrarySidebarTabV2', () => {
   it('should render with tabs', () => {
     const wrapper = mountComponent()
 
-    const triggers = wrapper.findAllComponents(TabsTrigger)
+    const triggers = wrapper.findAll('[role="tab"]')
     expect(triggers).toHaveLength(3)
   })