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.test.ts

@@ -50,8 +50,7 @@ const mountListView = (assetItems: OutputStackListItem[] = []) =>
       selectableAssets: [],
       isSelected: () => false,
       isStackExpanded: () => false,
-      toggleStack: async () => {},
-      assetType: 'output'
+      toggleStack: async () => {}
     },
     global: {
       stubs: {
@@ -61,18 +60,6 @@ const mountListView = (assetItems: OutputStackListItem[] = []) =>
   })
 
 describe('AssetsSidebarListView', () => {
-  it('shows generated assets header when there are assets', () => {
-    const wrapper = mountListView([buildOutputItem(buildAsset('a1', 'x.png'))])
-
-    expect(wrapper.text()).toContain('sideToolbar.generatedAssetsHeader')
-  })
-
-  it('does not show assets header when there are no assets', () => {
-    const wrapper = mountListView([])
-
-    expect(wrapper.text()).not.toContain('sideToolbar.generatedAssetsHeader')
-  })
-
   it('marks mp4 assets as video previews', () => {
     const videoAsset = {
       ...buildAsset('video-asset', 'clip.mp4'),