Match asset page layout and toolbar to shot page

- Remove wrapper padding from ProjectAssetsView so toolbar sits flush
  under the header, matching the shot page
- Add px-4 to AssetsDataTable root div to match ShotsDataTable margin
- Move thumbnail toggle from left filter group to right action area as
  an icon-only button, matching the shot toolbar's right-side pattern

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

frontend/src/views/project/ProjectAssetsView.vue

@@ -19,9 +19,7 @@
 
     <!-- Content -->
     <div class="flex-1 overflow-auto">
-      <div class="p-4 sm:p-6">
-        <AssetBrowser :project-id="projectId" />
-      </div>
+      <AssetBrowser :project-id="projectId" />
     </div>
   </div>
 </template>