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/AssetsSidebarListView.vue

@@ -1,19 +1,5 @@
 <template>
   <div class="flex h-full flex-col">
-    <div v-if="assetItems.length" class="px-2">
-      <div
-        class="flex items-center p-2 font-inter text-sm/normal font-normal text-muted-foreground"
-      >
-        {{
-          t(
-            assetType === 'input'
-              ? 'sideToolbar.importedAssetsHeader'
-              : 'sideToolbar.generatedAssetsHeader'
-          )
-        }}
-      </div>
-    </div>
-
     <VirtualGrid
       class="flex-1"
       :items="assetItems"
@@ -106,15 +92,13 @@ const {
   selectableAssets,
   isSelected,
   isStackExpanded,
-  toggleStack,
-  assetType = 'output'
+  toggleStack
 } = defineProps<{
   assetItems: OutputStackListItem[]
   selectableAssets: AssetItem[]
   isSelected: (assetId: string) => boolean
   isStackExpanded: (asset: AssetItem) => boolean
   toggleStack: (asset: AssetItem) => Promise<void>
-  assetType?: 'input' | 'output'
 }>()
 
 const assetsStore = useAssetsStore()