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/AssetsSidebarGridView.vue

@@ -1,20 +1,5 @@
 <template>
   <div class="flex h-full flex-col">
-    <!-- Assets Header -->
-    <div v-if="assets.length" class="px-2 2xl:px-4">
-      <div
-        class="flex items-center py-2 font-inter text-sm/normal font-normal text-muted-foreground"
-      >
-        {{
-          t(
-            assetType === 'input'
-              ? 'sideToolbar.importedAssetsHeader'
-              : 'sideToolbar.generatedAssetsHeader'
-          )
-        }}
-      </div>
-    </div>
-
     <!-- Assets Grid -->
     <VirtualGrid
       class="flex-1"
@@ -40,22 +25,14 @@
 
 <script setup lang="ts">
 import { computed } from 'vue'
-import { useI18n } from 'vue-i18n'
 
 import VirtualGrid from '@/components/common/VirtualGrid.vue'
 import MediaAssetCard from '@/platform/assets/components/MediaAssetCard.vue'
 import type { AssetItem } from '@/platform/assets/schemas/assetSchema'
 
-const {
-  assets,
-  isSelected,
-  assetType = 'output',
-  showOutputCount,
-  getOutputCount
-} = defineProps<{
+const { assets, isSelected, showOutputCount, getOutputCount } = defineProps<{
   assets: AssetItem[]
   isSelected: (assetId: string) => boolean
-  assetType?: 'input' | 'output'
   showOutputCount: (asset: AssetItem) => boolean
   getOutputCount: (asset: AssetItem) => number
 }>()
@@ -68,8 +45,6 @@ const emit = defineEmits<{
   (e: 'output-count-click', asset: AssetItem): void
 }>()
 
-const { t } = useI18n()
-
 type AssetGridItem = { key: string; asset: AssetItem }
 
 const assetItems = computed<AssetGridItem[]>(() =>