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/ModelLibrarySidebarTab.vue

@@ -21,7 +21,7 @@
       </Button>
     </template>
     <template #header>
-      <div class="px-2 2xl:px-4">
+      <SidebarTopArea>
         <SearchInput
           ref="searchBoxRef"
           v-model:model-value="searchQuery"
@@ -32,7 +32,7 @@
           "
           @search="handleSearch"
         />
-      </div>
+      </SidebarTopArea>
     </template>
     <template #body>
       <ElectronDownloadItems v-if="isDesktop" />
@@ -57,6 +57,7 @@ import { Divider } from 'primevue'
 import { computed, nextTick, onMounted, ref, toRef, watch } from 'vue'
 
 import SearchInput from '@/components/ui/search-input/SearchInput.vue'
+import SidebarTopArea from '@/components/sidebar/tabs/SidebarTopArea.vue'
 import TreeExplorer from '@/components/common/TreeExplorer.vue'
 import SidebarTabTemplate from '@/components/sidebar/tabs/SidebarTabTemplate.vue'
 import ElectronDownloadItems from '@/components/sidebar/tabs/modelLibrary/ElectronDownloadItems.vue'